I have been meaning to add this ability to the blockchain.info API for a while. You can now replace the guid in the send api (https://blockchain.info/api/api_send) with a Hex encoded private key.

https://blockchain.info/merchant/4d6c9dff493fcd2da9508e01c8b13461d37e3d8b6df1732942d3257874051362/payment?to=$address&amount=$amount

$address = destination bitcoin address.
$amount = amount to send in satoshi.