Well, first I like to thank burnin, this is awesome. I hope everyone tips this man once his asic is up and running.
but I need to know how to sign a message when I use an online wallet like blockchain. I paid for the chips with that ewallet, so no way I can just create an offline wallet or connect to this wallet via a client.. suggestions?
You could export your private key, import them to a bitcoin-qt client and then sign your message.
You can also presse "Send Money" and to the right of your adresse choose the Actions dropdown box and press sign message =