Board Thread:Episode Discussion/@comment-4010415-20171017224820/@comment-4010415-20171019233420

MetallicArcher wrote: Interaction with aura could alter the colour of the dust when activated, since auras have their own colours? It's possible, but considering that Ruby, Weiss, and Blake have all three ended up with a black-and-purple effect, I'm not sure it's from Aura color altering the color of the Dust effect.

Plus, while those massive crystals on the sky islands are black, Weiss doesn't have any black powdered Dust in Myrtenaster, and she didn't give any to Blake either. Instead, there was purple.