As far I as know you can only be signed message on blockchain.info with your imported address. But as I tried to access my blockchain.info account and tried to find the signing message but it seems that they removed this feature now. But the verifying message is still there, dunno if what's the reason why they removed it without notifying their users.
EDIT: Okay I found it! Blockchain.info hide this feature. This might be complicated but working.
Must use a desktop/PC, Go to Settings > Addresses > Scroll Down > Choose the address you want to signed message, Click the "More Options" then Right click the "Spend" then click "Inspect Element"
You will see like this
Then click the 4th one, and you will see like this
Then double click this "ng-hide" on the class and delete it, now click anywhere on the console and the "Sign Message" button will now appear on the dropdown. And you can try to signed message by clicking the button if it will work.