A deposit insurance is a measure implemented in many countries to protect bank depositors, in full or in part, from losses caused by a bank's inability to pay its debts when due.
Deposit insurance systems are one component of a financial system safety net that promotes financial stability. In Germany the deposit insurance covers losses of up to 100,000 EUR meaning that your blocked balance will always be insured on the basis of German law.
Note: of course, this does not impose any additional costs to you or your blocked account. The deposit insurance is already included and free of charge for every account opening at any bank in Germany.