User blog comment:DiegoAnime3000/Black is for Polarity/@comment-11346598-20140111013324/@comment-3257939-20140111021158

I don't think outfits have anything to do with dust, aura or semblance. Remember, Pyrrha wears red too and her semblance is polarity. Blake wears black and her semblance is shadows.

But Ruby's red cloak definitely has something to do with her semblance, she only takes it off when she goes to sleep.