I am new to BTC and I have a question: How to get private key from Blockchain wallet for a "Watch only" address? I have been trying for days with no positive results. Thanks.
You have to have that private key already. You cannot derive it from the public address. A watch-only address is an imported address.
If you've imported it from a paper wallet, check for the private key that should be printed on the same paper. You can scan the private key and upgrade the watch-only address to one that you can spend from as well.
If you imported some address you do not have the private key of, it will remain a watch-only address for you forever.
How did you add your address to blockchain.info? Did you use a paper wallet Qrcode to import your address? In this case, you supposed to write your private key and if you haven't you've lost it.
As I remember if you just used blockchai.info to have a wallet, blockchain.info won't give you any private key.