Raseri, sorry, had to do some stuff. Anywho, Black you say?
Take a Cottonee(Whew, starting far away, eh?). It must have the ability "Prankster" Now, let's look at Whimsicott. It's pokedex data for White states...
Riding whirlwinds, they appear. These pokemon sneak through gaps into houses and cause all sorts of mischief.
Thus, we can assume our Cottonee dreams of being a prankster. Thus, it must learn a type of powder to mess people with(Poisonpowder or stun spore either one). Also, since it is a GIANT COTTONBALL, it must know Cotton Spore. Since it rides whirlwinds, it must know Hurricane, and finally, since it causes mischief, it must know either Solarbeam or Energy Ball to smash things with.
Nickname it "Stealthy", because pranksters have to be that way, am I right?