User blog comment:DiegoAnime3000/Black is for Polarity/@comment-10380701-20140110101923

Given that, there has to be some black or dark grey Dust in order for Weiss to be able to perform that with her glyphs. Pyrrha's semblance IS polarity, so she doesn't require Dust with that.

I'm calling that it's the gray-ish one next to the red one.