Download the Bitcoin Core wallet: signing addresses with that wallet is very easy.
For any doubt or information feel free to ask.
Best regards.
But we need a signature from the address that user has previously used (and posted in a PM).
Hence only the 131pB address comes to mind.
I was giving advices.
However, if the user is unabled to export the private key from blockchain.info (or get to it in any other way),
downloading core won't be that much a help with this task.
That is true.
Best regards.