can they be hacked
Anything on the internet can be hacked. Once you know what you're doing, hacking into company websites and computers is a cinch.
Are you sure with that?
You can't literally hacked all of the things in the internet thus you can collect information through bypassing a security. That's the reason why some of the social media accounts where being hacked but literally they're being hijacked and the information was being collected. Also, blockchain has an ultimate security, you can't easily hacked each wallet and control the transaction. The only thing you can do is decrypt but that's not very sure you can do it.
there is no "ultimate security" when it comes to security and every security setup can be broken into. so yeah blockchain.info like anything else in computer world can be hacked, it is just different levels of hardship.
they keep wallets encrypted in their servers and the process of encryption/decryption is done inside user's browser so if you hack their server you can gain encrypted wallet files. but the encryption can be broken too. for example if their implementation had some bug in it, considering blockchain.info has a lot of bugs in their system it is not far fetched to think there are security holes everywhere else too.