Hi Alan and welcome to NGA!
Sounds like you have Sooty Mold, Fumago vagens. It's a fungus that grows on the droppings of insects like aphids that sucks the sap from plants. The fix is get rid of the bugs then apply a fungicide to kill the mold. If there aren't too many leaves then you can wipe it off with a damp paper towel but you'll still need to get rid of the bugs.
