Remove the brake calliper (so you don't hear the pads on the disc) and spin the wheel, if there is a grumble its a sign of impending failure. If there is just a whirring sound then I would ride for a while longer and watch the magnet. Typically there are more/larger flakes than you have.
Any black fluff is perfectly normal.
BTW: I am assuming you have an Oilhead right?
It helps to fill out your profile.