User blog comment:Sorairo/Aura, Dust, and the RWBY Panel/@comment-2135507-20130823054611

I'm betting Adam is using the darkness in his soul for his aura and that's why he wears a grim-like mask. He's slowly turning into one. Blake saw that there was nothing more she could do so she decided to leave him. He may have also been influencing her to use the darkness in her soul as well.

Also it seems Dust is just a substance that can manifest your aura in a more potent manner maybe?