Originally Posted by
clengman
Brake dust. You can clean it off, but it'll be back the next time you ride your bike. Not much you can do about it.
I agree it looks like rubber grime from the brakes. You get the same looking grime if you wet your fingers and run them along your rubber windshield wipers on your car.
But how would it get on the inside of the rim near the spokes? The outward force of the spinning wheel should push anything from the brakes out towards the tire.