After you beat the game once, the Black Joe will start challenging you to one-on-one races. When you defeat him, you’ll face off with the Black Roader and six of his buddies, and then once they admit defeat you’ll be challenged by the Black Devil (black version of the Rock Monster, otherwise known as the Yellow Devil). Then finally you’ll face all three at once (known as the Black Troop). Each of these will appear in the Grand Prix mode as a “new challenger” during one of the normal races (yes, even against Wily). If you choose to race them you’ll end up competing on whatever track you’d happened to be racing at that point. You only get one chance each time you are challenged; unlike the other races, you cannot “Restart” and try again. If you lose, you have to wait until you’re challenged again in order to try again.