Actually, now that I think about it, I do clean the rims about 1-2x week, usually with a clean, dry towel. But every other week with Windex. I do this mostly for cosmetics-- i.e.,spokes, wheels, but the braking-rim surface does get some love in this ritual.
Still, with my recent deliberate cleaning of rim (brake-pad mating area), I did pull off black stuff. May've been partly the 90% alch. towel rubbing the tire???