^Blockchain hasn't changed their design. Just login to your account, click Receive Money tab, next to your address there will be a Actions drop-down box which you need to click and select Sign Message. In the pop up, type a message to verify your account and copy paste the code you get.
they have a new wallet. there's an option to switch to the new wallet on the log in page. once you switch you can't go back and you're stuck with an incomplete wallet with
no option to either sign a message or export private keys.
if OP have saved the private key of that address somewhere he can import that to another wallet. if he can't then hopefully theymos will accept sending bitcoins from that address as proof.