Company: MYI
Filing Date: 2025-09-05
Form Type: 424B3
Source: 0001193125-25-196285
Chunk: 30

Company: BLACKROCK MUNIYIELD QUALITY FUND III, INC.
Filing Date: 2025-09-05
Form: 424B3
Chunk 30
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 control number and security code that will allow you to vote at the Special Meeting. Even if you plan to attend the Special Meeting, please promptly follow the enclosed instructions to submit voting instructions by telephone or via the Internet. Alternatively, you may submit voting instructions by signing and dating each proxy card you receive, and if received by mail, returning it in the accompanying postage-paid return envelope.

| Q: | Whom do I contact for further information? |

| A: | You may contact your financial advisor for further information. You may also call Georgeson, the 
 Funds’ proxy solicitor, at 1-888-686-6309.                                                       |

| Q: | Will anyone contact me? |

| A: | You may receive a call from Georgeson, the proxy solicitor hired by the Funds, to verify that you received              
 your proxy materials, to answer any questions you may have about the proposals and to encourage you to vote your proxy. |

We recognize the inconvenience of the proxy solicitation process and would not impose on you if we did not believe that the matters being proposed were important. Once your vote has been registered with the proxy solicitor, your name will be removed from the solicitor’s follow-upcontact list. Your vote is very important. We encourage you as a shareholder to participate by returning your vote as soon as possible. If enough shareholders fail to cast their votes, a Fund may not be able to hold the Special Meeting or the vote on the applicable proposals, and will be required to incur additional solicitation costs in order to obtain sufficient shareholder participation. Important additional information about the Reorganizations is set forth in the accompanying Joint Proxy Statement/Prospectus. Please read it carefully. xvi

BLACKROCK MUNIVEST FUND II, INC.

BLACKROCK MUNIYIELD MICHIGAN QUALITY FUND, INC.

BLACKROCK MUNIVEST FUND, INC.

BLACKROCK MUNIYIELD QUALITY FUND III, INC.

100 Bellevue Parkway

Wilmington, Delaware 19809

(800) 882-0052

NOTICE OF JOINT SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD ON OCTOBER 15, 2025

Notice is hereby given that a joint special meeting of shareholders (the “Special Meeting”) of BlackRock MuniVest Fund II, Inc.
(NYSE Ticker: MVT) (“MVT”), BlackRock MuniYield Michigan Quality Fund, Inc