Blank money orders are like cash. Anyone may use them. However, was the receipt still attached? If it wasn't, the person who bought the money order may have since reported it lost or stolen, and I'm not familiar with the legal issues that may arise if you cash it.
There is no such thing as a blank money order .it has to be stolen. Money orders are time stamped and computer logged from the seller/agent. And have to be signed by the purchaser for the receiver to cash them.