How can I sign message for a blockchain watch only bitcoin address
When you import an address as a 'watch only' address, it means exactly that: you can only 'watch' the address and see whatever funds are sitting in it. You cannot send transactions, or sign messages, with a watch only address. 

This is because you just imported the address into your wallet, whereas if you imported the private key, then you would be able to sign the message (and send transactions, etc).

So you'll need to find some wallet software that can sign messages (and that you trust). Import the private key for that address, and then sign the message.