The good thing about that bright white 4k side is that you can see this reeeeeal easy. I know you said you were using tape on the W&B, but if you were on the DD too, the only thing touching the stone, as far as bare metal, should have been the bevel. If it was still leaving black marks, you definitely have more work to do there.
I don't know how this myth originated..The abraded particles don't magically get bleached.
If you rub steel on a white stone it will leave black marks.