Most of the blocked account providers do not let you transfer money out of the account whenever you want, after activating the account when you arrive in Germany you can set up a monthly standing order with your bank to get access to your funds.
If you need more money than the available amount in your blocked account, you’ll need to go to a local immigration office and ask for an account release approval. They’ll give you a document that you can submit to your banking provider to release the extra funds.