Thank you, that is pretty much what I was looking for when I start the thread. I am still trying to think how to use that api to sign by private key without storing private key on the app server.
You don't have to store the private key on your server if you use blockchain's API. Of course you'll need to provide your blockchain wallet ID and password, but you can just store a limited amount on blockchain and keep your "real" bitcoin balance in cold storage. Then just transfer from cold storage to your hot wallet on blockchain API when needed.