Hello Lutz,
Please understand the fundamentals of blocked stock as below.
Blocked stock come while quality rejection or during sales return activity & blocked stock means fundamentally that we might not use this till it is not checked/verified by Quality person.
If you know that this stock is not Rejected and can be use, then you need to do manual stock transfer via MB1B or MIGO with Movement type 344.
Cheers!
Neeraj Dixit