Thanks for guiding.
BTW I am using Blockchain wallet.
Someone right me if I'm wrong, but Blockchain doesn't let you export the private keys. Nevertheless, since you need to write the seed down (12 word phrase), you should be able to use that seed on another wallet that allows private key exporting (Electrum), and from there, you can get the private key
Again, someone right me if I'm wrong please
You are correct, blockchain wallet gives seed key or phrase key. OP have to write down the key as it is maintaining the order and later import in electrum wallet. Then follow my above suggestion to get private key of certain address. However, I recommend OP not to using blockchain wallet since it's an web wallet.