the only thing I've found for dark fabric is the slivers of soap that are left when the bar is almost gone and it has the sharp edges on it. Those work well for doing small areas as you go. I grab them out of the shower when they are dry and toss into a box. Oh yeah, those pencils are infuriating when the lead keeps breaking. I'm going to try a mechanical pencil with white or yellow lead on my current project.