I would not ever use online wallets like blockchain for storing any substantial amount, they are a target for hackers and their security can be compromised. Or the service operator may decide one day, that it's more profitable to steal all clients wallets, than run the service.
In the case of blockchain.info the private keys are stored locally on your computer and they don't have access to them so it's way safer than other online wallets.
But still, there are lots of risks associated with using them. No online wallet is 100% safe.
The only safe wallet is an offline wallet properly created.