I need to correct my earlier post about drop rates: black dragons (incl. KBD) have a drop rate of around 1/2000 and only QBD is 1/200. Sorry about that, I misread Mod Rowley's post on Reddit.
And I also have extra info: Studly is better for breeding than Virile. Percentage is not clear for now but if you're wavering between keeping a studly or a virile animal, keep the studly one.