How can I sign message for a blockchain watch only bitcoin address I have to verify sign message for one of my blockchain bitcoin address. I couldn't find sign message or private key option in More Options beside all my bitcoin addresses. I thought maybe I have to import it.So I imported my own blockchain bitcoin address which I am mentioning below

1PSBpnjrzpHyKpyQuFaecdnA4WfDetdiwn

and it became a watch only address. In More Options I see only Archive, transfer and spent.
So what to do now ? Please help...

I do not have any bitcoins on this address, but I logged in using this bitcoin address as master address on a bitcoin gambling site and forgot the password.The only way I can recover my account is by verifying my bitcoin address 

1PSBpnjrzpHyKpyQuFaecdnA4WfDetdiwn and pasting the signature to be provided by blockchain. I don't want to lose my BTC and LTC on this account.

So what to do now ? Please help...
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.