It seems more of a recessive gene that makes the Yellow Jacket that could have some heterogeneous markings occasionally. Kind of like the Het-Pied markings, they are not always true Het. YJ's.

So really, consider this a recessive gene for YJ's and you buying Het. YJ, not black belly. Or one day you may think you are buying a 'black belly' that looks like a Het. YJ but really is not.

Just my thoughts on the matter.