Have you tried a Q-tip with a wee dab of alcohol or latex-paint solvent on it? I've found that sometimes works, but I found out the hard way to test on an obscure seam first, as sometimes the color itself will come out with alcohol.... The latex-paint solvent (I use "Oops") usually works for me for just about any type of stain. If you cannot get all the white marks out, I'd try using a black permanent marker designed for fabric only--not just a permanent general-purpose marker--broad tip, and very lightly cover them up. These markers can do wonderful things on black fabric, the ink is permanent, and it, if you use it just so, blends right into the original color. It does not compromise the piece at all. I touched up a dress recently at the underam area, and even I can't tell where I did it.... Of course, this wouldn't work for a very large area, but for what you are describing, I'd say it would be a suitable use!