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.
Electrum v 2.9.3
Open Electrum, go to "addresses" then copy the address you want to sign.
Now click on "tools" and then "sign/verify message" from the drop down menu.
Enter your message in the first box, paste the address you want to sign in the provided field and hit "sign"
You will now have the signature string displayed below the bitcoin address.
Mycelium (android wallet)
Open the Mycelium wallet app and go to the "Accounts" page (unlock if needed)
Click on the Account that says "contains private keys" then Click on the 3 little dots in the top right corner and select "sign message"
Enter your pin again to unlock the address list, click on the address you want to sign and this will open the page for you to sign the address.
Once you have signed the address copy the signature and text to your clipboard.