specifically for blockchain.info, yes, you can store them in an address they supply to you (they just won't be able to see the tokens). that's ok because they give you clear text access to your private keys.
in order to send the tokens, one need to import the private key to an omni wallet.
the availability of the privkey is therefore the only criterion for storing omni/xcp tokens in a regular bitcoin wallet
I sent 1 test token to my omni wallet, and the only place it is listed is under History and shown as a Smart Property. However, when I try to send it back to bittrex, it gives me the error "Cannot send anything because the wallet has no coins in the addresses that have private keys (no coins)". How exactly do I send the tokens?