I have never tried this to see if it works, but I found a tip somewhere on the net that says peanut butter will get Crayola off a blackboard. I don't know what you are supposed to do with it or whether crunchy or smooth will make a difference :) I don't suppose rubbing a little bit on will harm the blackboard anymore
This will sound crazy and I have no idea if it will work...
To get candle wax off fabric I use brown paper and an iron. The wax melts into the paper and comes away from the fabric.
But would that work on a blackboard...