Sign into your Blockchain.info wallet
Go into the "Settings" option and click on "Addresses" from the list.
Once you are in the "Addresses" field you will see a list of all the imported addresses you wallet has the private keys to. Please note that you will only be able to sign Import Bitcoin Address from the old Legacy wallet.
Find the address you want to sign, click on "more options" to the right of the address and then select "sign message"
You should now have a box appear for you to sign the bitcoin address you have selected, enter your message and click "sign" you should now see the signature string appear.