Block io

Block.io provides zero fee transactions for users, developers, and merchants. However, we do not deal in money.

Block.io uses MultiSig addresses that require two signatures for every withdrawal/transaction: yours, and Block.io's. This means that Block.io has no control over your coins, and cannot move your coins without your signatures. This also means that in the event that Block.io is compromised, none of your coins can be lost because of just that event. MultiSig addresses exponentially increase users' wallet security.
If a sending wallet/service says the MultiSig addresses at Block.io are invalid, this means the sending service does not support MultiSig addresses. It is very likely that the service is insecure, and outdated. We recommend you stop using that site or service immediately, and contact the developer for comment.