I have unmachined Deep V's with a brake on the front, that black mark is going to be there until it rubs off from the brakes, and by that point there will be other black marks. Then you will see the brakes start to eat through the paint/glaze (what ever yours are coated with). Then eventually it will just be silver and shinny. Might as well get over it and feel comfortable about getting your rims dirty from now on.