I got caught in a rain storm yesterday, noticed my rear brake wasn't catching well in the rain, got home and dried everything off and the rear wheel/dropouts were completely black-streaked, covered in some kind of oily sludge. Tried to dissolve it with rubbing alcohol but there wasn't enough. The pads look to have been fouled so I'm going to swap those out (I don't think washing them will do the trick, will it?). I re-lubed the chain twice and the RD. I'm going to hose down and scrub everything this weekend, but where on earth did all this oil come from?!
This stuff obviously came from my bike, since the front wheel is fine. I don't think I ran through any oil puddles although that's a possibility.