Exhibit 4(d)


                        SUB-INVESTMENT ADVISORY AGREEMENT

      AGREEMENT dated October 2, 2006, between BlackRock Advisors, LLC, a
Delaware limited liability corporation (the "Advisor"), and BlackRock Investment
Management, LLC, a Delaware limited liability corporation (the "Sub-Advisor").

      WHEREAS, the Advisor has agreed to furnish investment advisory services to
the BlackRock High Yield Municipal Fund (the "Fund"), a series of BlackRock
Municipal Bond Fund, Inc., a Maryland corporation (the "Corporation"), an
open-end management investment company registered under the Investment Company
Act of 1940, as amended (the "1940 Act");

      WHEREAS, the Advisor wishes to retain the Sub-Advisor to provide it with
certain sub-advisory services as described below in connection with Advisor's
advisory activities on behalf of the Fund;

      WHEREAS, the advisory agreement between the Advisor and the Corporation,
dated October 2, 2006 (such agreement or the most recent successor agreement
between such parties relating to advisory services to the Corporation is
referred to herein as the "Advisory Agreement") contemplates that the Advisor
may sub-contract investment advisory services with respect to the Fund to a
sub-advisor pursuant to a sub-advisory agreement agreeable to the Corporation
and approved in accordance with the provisions of the 1940 Act; and

      WHEREAS, this Agreement has been approved in accordance with the
provisions of the 1940 Act, and the Sub-Advisor is willing to furnish such
services upon the terms and conditions herein set forth;

      NOW, THEREFORE, in consideration of the mutual premises and covenants
herein contained and other good and valuable consideration, the receipt of which
is hereby acknowledged, it is agreed by and between the parties hereto as
follows:

      1. Appointment. The Advisor hereby appoints the Sub-Advisor to act as
sub-advisor with respect to the Fund and the Sub-Advisor accepts such
appointment and agrees to render the services herein set forth for the
compensation herein provided.

      2. Services of the Sub-Advisor. Subject to the succeeding provisions of
this section, the oversight and supervision of the Advisor and the direction and
control of the Corporation's Board of Directors, the Sub-Advisor will perform
certain of the day-to-day operations of the Fund, which may include one or more
of the following services, at the request of the Advisor: (a) acting as
investment advisor for and managing the investment and reinvestment of those
assets of the Fund as the Advisor may from time to time request and in
connection therewith have complete discretion in purchasing and selling such
securities and other assets for the Fund and in voting, exercising consents and
exercising all other rights appertaining to such securities and other assets on
behalf of the Fund; (b) arranging, subject to the provisions of paragraph 3
hereof, for the purchase and sale of securities and other assets of the Fund;
(c) providing investment research and credit analysis concerning the Fund's


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investments, (d) assist the Advisor in determining what portion of the Fund's
assets will be invested in cash, cash equivalents and money market instruments,
(e) placing orders for all purchases and sales of such investments made for the
Fund, and (f) maintaining the books and records as are required to support Fund
investment operations. At the request of the Advisor, the Sub-Advisor will also,
subject to the oversight and supervision of the Advisor and the direction and
control of the Corporation's Board of Directors, provide to the Advisor or the
Fund any of the facilities and equipment and perform any of the services
described in Section 3 of the Advisory Agreement. In addition, the Sub-Advisor
will keep the Fund and the Advisor informed of developments materially affecting
the Fund and shall, on its own initiative, furnish to the Fund from time to time
whatever information the Sub-Advisor believes appropriate for this purpose. The
Sub-Advisor will periodically communicate to the Advisor, at such times as the
Advisor may direct, information concerning the purchase and sale of securities
for the Fund, including: (a) the name of the issuer, (b) the amount of the
purchase or sale, (c) the name of the broker or dealer, if any, through which
the purchase or sale is effected, (d) the CUSIP number of the instrument, if
any, and (e) such other information as the Advisor may reasonably require for
purposes of fulfilling its obligations to the Fund under the Advisory Agreement.
The Sub-Advisor will provide the services rendered by it under this Agreement in
accordance with the Fund's investment objectives, policies and restrictions (as
currently in effect and as they may be amended or supplemented from time to
time) as stated in the Fund's Prospectus and Statement of Additional Information
and the resolutions of the Corporation's Board of Directors.

      3. Covenants. (a) In the performance of its duties under this Agreement,
the Sub-Advisor shall at all times conform to, and act in accordance with, any
requirements imposed by: (i) the provisions of the 1940 Act and the Investment
Advisers Act of 1940, as amended (the "Advisers Act") and all applicable Rules
and Regulations of the Securities and Exchange Commission (the "SEC"); (ii) any
other applicable provision of law; (iii) the provisions of the Charter and
By-Laws of the Corporation, as such documents are amended from time to time;
(iv) the investment objectives and policies of the Fund as set forth in the
Corporation's Registration Statement on Form N-1A and/or the resolutions of the
Board of Directors; and (v) any policies and determinations of the Board of
Directors of the Corporation and

            (b) In addition, the Sub-Advisor will:

                  (i) place orders either directly with the issuer or with any
            broker or dealer. Subject to the other provisions of this paragraph,
            in placing orders with brokers and dealers, the Sub-Advisor will
            attempt to obtain the best price and the most favorable execution of
            its orders. In placing orders, the Sub-Advisor will consider the
            experience and skill of the firm's securities traders as well as the
            firm's financial responsibility and administrative efficiency.
            Consistent with this obligation, the Sub-Advisor may select brokers
            on the basis of the research, statistical and pricing services they
            provide to the Fund and other clients of the Advisor or the
            Sub-Advisor. Information and research received from such brokers
            will be in addition to, and not in lieu of, the services required to
            be performed by the Sub-Advisor hereunder. A commission paid to such
            brokers may be higher than that which another qualified broker would
            have charged for effecting the same transaction, provided that the
            Sub-


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            Advisor determines in good faith that such commission is reasonable
            in terms either of the transaction or the overall responsibility of
            the Advisor and the Sub-Advisor to the Fund and their other clients
            and that the total commissions paid by the Fund will be reasonable
            in relation to the benefits to the Fund over the long-term. Subject
            to the foregoing and the provisions of the 1940 Act, the Securities
            Exchange Act of 1934, as amended, and other applicable provisions of
            law, the Advisor may select brokers and dealers with which it or the
            Corporation is affiliated;

                  (ii) maintain books and records with respect to the Fund's
            securities transactions and will render to the Advisor and the
            Corporation's Board of Directors such periodic and special reports
            as they may request;

                  (iii) maintain a policy and practice of conducting its
            investment advisory services hereunder independently of the
            commercial banking operations of its affiliates. When the
            Sub-Advisor makes investment recommendations for the Fund, its
            investment advisory personnel will not inquire or take into
            consideration whether the issuer of securities proposed for purchase
            or sale for the Fund's account are customers of the commercial
            department of its affiliates; and

                  (iv) treat confidentially and as proprietary information of
            the Fund all records and other information relative to the Fund, and
            the Fund's prior, current or potential shareholders, and will not
            use such records and information for any purpose other than
            performance of its responsibilities and duties hereunder, except
            after prior notification to and approval in writing by the Fund,
            which approval shall not be unreasonably withheld and may not be
            withheld where the Sub-Advisor may be exposed to civil or criminal
            contempt proceedings for failure to comply, when requested to
            divulge such information by duly constituted authorities, or when so
            requested by the Fund.

      4. Services Not Exclusive. Nothing in this Agreement shall prevent the
Sub-Advisor or any officer, employee or other affiliate thereof from acting as
investment advisor for any other person, firm or corporation, or from engaging
in any other lawful activity, and shall not in any way limit or restrict the
Sub-Advisor or any of its officers, employees or agents from buying, selling or
trading any securities for its or their own accounts or for the accounts of
others for whom it or they may be acting; provided, however, that the
Sub-Advisor will undertake no activities which, in its judgment, will adversely
affect the performance of its obligations under this Agreement.

      5. Books and Records. In compliance with the requirements of Rule 31a-3
under the 1940 Act, the Sub-Advisor hereby agrees that all records which it
maintains for the Corporation are the property of the Corporation and further
agrees to surrender promptly to the Corporation any such records upon the
Corporation's request. The Sub-Advisor further agrees to preserve for the
periods prescribed by Rule 31a-2 under the 1940 Act the records required to be
maintained by Rule 31a-1 under the 1940 Act (to the extent such books and
records are not maintained by the Advisor).


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      6. Expenses. During the term of this Agreement, the Sub-Advisor will bear
all costs and expenses of its employees and any overhead incurred by the
Sub-Advisor in connection with its duties hereunder; provided that the Board of
Directors of the Corporation may approve reimbursement to the Sub-Advisor of the
pro-rata portion of the salaries, bonuses, health insurance, retirement benefits
and all similar employment costs for the time spent on Fund operations
(including, without limitation, compliance matters) (other than the provision of
investment advice and administrative services required to be provided hereunder)
of all personnel employed by the Sub-Advisor who devote substantial time to Fund
operations or the operations of other investment companies advised or
sub-advised by the Sub-Advisor.

      7. Compensation.

            (a) The Advisor agrees to pay to the Sub-Advisor and the Sub-Advisor
      agrees to accept as full compensation for all services rendered by the
      Sub-Advisor as such, a monthly fee in arrears at an annual rate equal to
      the amount set forth in Schedule A hereto. For any period less than a
      month during which this Agreement is in effect, the fee shall be prorated
      according to the proportion which such period bears to a full month of 28,
      29, 30 or 31 days, as the case may be.

            (b) For purposes of this Agreement, the net assets of the Fund shall
      be calculated pursuant to the procedures adopted by resolutions of the
      Directors of the Corporation for calculating the value of the Fund's
      assets or delegating such calculations to third parties.

      8. Indemnity.

            (a) The Corporation may, in the discretion of the Board of Directors
      of the Corporation, indemnify the Sub-Advisor, and each of the
      Sub-Advisor's directors, officers, employees, agents, associates and
      controlling persons and the directors, partners, members, officers,
      employees and agents thereof (including any individual who serves at the
      Sub-Advisor's request as director, officer, partner, member, trustee or
      the like of another entity) (each such person being an "Indemnitee")
      against any liabilities and expenses, including amounts paid in
      satisfaction of judgments, in compromise or as fines and penalties, and
      counsel fees (all as provided in accordance with applicable state law)
      reasonably incurred by such Indemnitee in connection with the defense or
      disposition of any action, suit or other proceeding, whether civil or
      criminal, before any court or administrative or investigative body in
      which such Indemnitee may be or may have been involved as a party or
      otherwise or with which such Indemnitee may be or may have been
      threatened, while acting in any capacity set forth herein or thereafter by
      reason of such Indemnitee having acted in any such capacity, except with
      respect to any matter as to which such Indemnitee shall have been
      adjudicated not to have acted in good faith in the reasonable belief that
      such Indemnitee's action was in the best interest of the Corporation and
      furthermore, in the case of any criminal proceeding, so long as such
      Indemnitee had no reasonable cause to believe that the conduct was
      unlawful; provided, however, that (1) no Indemnitee shall be indemnified
      hereunder against any liability to the Corporation or its shareholders or
      any expense of such Indemnitee arising by reason of (i) willful
      misfeasance, (ii) bad faith, (iii) gross negligence or (iv) reckless
      disregard of the duties involved in the conduct of such Indemnitee's
      position (the conduct referred to in such clauses (i) through (iv)


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      being sometimes referred to herein as "disabling conduct"), (2) as to any
      matter disposed of by settlement or a compromise payment by such
      Indemnitee, pursuant to a consent decree or otherwise, no indemnification
      either for said payment or for any other expenses shall be provided unless
      there has been a determination that such settlement or compromise is in
      the best interests of the Corporation and that such Indemnitee appears to
      have acted in good faith in the reasonable belief that such Indemnitee's
      action was in the best interest of the Corporation and did not involve
      disabling conduct by such Indemnitee and (3) with respect to any action,
      suit or other proceeding voluntarily prosecuted by any Indemnitee as
      plaintiff, indemnification shall be mandatory only if the prosecution of
      such action, suit or other proceeding by such Indemnitee was authorized by
      a majority of the full Board of Directors of the Corporation.

            (b) The Corporation shall make advance payments in connection with
      the expenses of defending any action with respect to which indemnification
      might be sought hereunder if the Corporation receives a written
      affirmation of the Indemnitee's good faith belief that the standard of
      conduct necessary for indemnification has been met and a written
      undertaking to reimburse the Corporation unless it is subsequently
      determined that such Indemnitee is entitled to such indemnification and if
      the Directors of the Corporation determine that the facts then known to
      them would not preclude indemnification. In addition, at least one of the
      following conditions must be met: (A) the Indemnitee shall provide a
      security for such Indemnitee undertaking, (B) the Fund shall be insured
      against losses arising by reason of any unlawful advance, or (C) a
      majority of a quorum consisting of Directors of the Corporation who are
      neither "interested persons" of the Corporation (as defined in Section
      2(a)(19) of the 1940 Act) nor parties to the proceeding ("Disinterested
      Non-Party Directors") or an independent legal counsel in a written
      opinion, shall determine, based on a review of readily available facts (as
      opposed to a full trial-type inquiry), that there is reason to believe
      that the Indemnitee ultimately will be found entitled to indemnification.

            (c) All determinations with respect to the standards for
      indemnification hereunder shall be made (1) by a final decision on the
      merits by a court or other body before whom the proceeding was brought
      that such Indemnitee is not liable by reason of disabling conduct, or (2)
      in the absence of such a decision, by (i) a majority vote of a quorum of
      the Disinterested Non-Party Directors of the Corporation, or (ii) if such
      a quorum is not obtainable or even, if obtainable, if a majority vote of
      such quorum so directs, independent legal counsel in a written opinion.
      All determinations that advance payments in connection with the expense of
      defending any proceeding shall be authorized shall be made in accordance
      with the immediately preceding clause (2) above.

      The rights accruing to any Indemnitee under these provisions shall not
exclude any other right to which such Indemnitee may be lawfully entitled.

      9. Limitation on Liability. The Sub-Advisor will not be liable for any
error of judgment or mistake of law or for any loss suffered by the Advisor or
by the Fund in connection with the performance of this Agreement, except a loss
resulting from a breach of fiduciary duty with respect to the receipt of
compensation for services or a loss resulting from willful misfeasance, bad
faith or gross negligence on its part in the performance of its duties or from
reckless disregard by it of its duties under this Agreement. As used in this
Section 9(a), the term "Sub-Advisor" shall include any


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affiliates of the Sub-Advisor performing services for the Fund contemplated
hereby and partners, directors, officers and employees of the Sub-Advisor and
such affiliates.

      10. Duration and Termination. This Agreement shall become effective as of
the date hereof and, unless sooner terminated with respect to the Fund as
provided herein, shall continue in effect for a period of two years. Thereafter,
if not terminated, this Agreement shall continue in effect with respect to the
Fund for successive periods of 12 months, provided such continuance is
specifically approved at least annually by both (a) the vote of a majority of
the Corporation's Board of Directors or a vote of a majority of the outstanding
voting securities of the Fund at the time outstanding and entitled to vote and
(b) by the vote of a majority of the Directors, who are not parties to this
Agreement or interested persons (as such term is defined in the 1940 Act) of any
such party, cast in person at a meeting called for the purpose of voting on such
approval. Notwithstanding the foregoing, this Agreement may be terminated by the
Corporation or the Advisor at any time, without the payment of any penalty, upon
giving the Sub-Advisor 60 days' notice (which notice may be waived by the
Sub-Advisor), provided that such termination by the Corporation or the Advisor
shall be directed or approved by the vote of a majority of the Directors of the
Corporation in office at the time or by the vote of the holders of a majority of
the voting securities of the Fund at the time outstanding and entitled to vote,
or by the Sub-Advisor on 60 days' written notice (which notice may be waived by
the Corporation and the Advisor), and will terminate automatically upon any
termination of the Advisory Agreement between the Corporation and the Advisor.
This Agreement will also immediately terminate in the event of its assignment.
(As used in this Agreement, the terms "majority of the outstanding voting
securities," "interested person" and "assignment" shall have the same meanings
of such terms in the 1940 Act.)

      11. Notices. Any notice under this Agreement shall be in writing to the
other party at such address as the other party may designate from time to time
for the receipt of such notice and shall be deemed to be received on the earlier
of the date actually received or on the fourth day after the postmark if such
notice is mailed first class postage prepaid.

      12. Amendment of this Agreement. This Agreement may be amended by the
parties only if such amendment is specifically approved by the vote of the Board
of Directors of the Corporation, including a majority of those Directors who are
not parties to this Agreement or interested persons of any such party cast in
person at a meeting called for the purpose of voting on such approval and, where
required by the 1940 Act, by a vote of a majority of the outstanding voting
securities of the Fund.

      13. Miscellaneous. The captions in this Agreement are included for
convenience of reference only and in no way define or delimit any of the
provisions hereof or otherwise affect their construction or effect. If any
provision of this Agreement shall be held or made invalid by a court decision,
statute, rule or otherwise, the remainder of this Agreement shall not be
affected thereby. This Agreement shall be binding on, and shall inure to the
benefit of the parties hereto and their respective successors.


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      14. Governing Law. This Agreement shall be governed by and construed in
accordance with the laws of the State of New York for contracts to be performed
entirely therein without reference to choice of law principles thereof and in
accordance with the applicable provisions of the 1940 Act. To the extent that
the applicable laws of the State of New York, or any of the provisions, conflict
with the applicable provisions of the 1940 Act, the latter shall control.

      15. Counterparts. This Agreement may be executed in counterparts by the
parties hereto, each of which shall constitute an original counterpart, and all
of which, together, shall constitute one Agreement.

      IN WITNESS WHEREOF, the parties hereto have caused this instrument to be
executed by their duly authorized officers designated below as of the day and
year first above written.

                                    BLACKROCK ADVISORS, LLC


                                    By:
                                        ----------------------------------------
                                        Name:
                                        Title:


                                    BLACKROCK INVESTMENT MANAGEMENT, INC.


                                    By:
                                        ----------------------------------------
                                        Name:
                                        Title:

AGREED AND ACCEPTED
as of the date first set forth above

BLACKROCK MUNICIPAL BOND FUND, INC.

By:
   -----------------------------
Name:
Title:


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                                   Schedule A

                           Sub-Investment Advisory Fee

_____% of the monthly advisory fee received by the Advisor from the Fund.