User blog comment:Sorairo/Aura, Dust, and the RWBY Panel/@comment-2135507-20130823054611/@comment-15108745-20130823055759

Well, the Grim don't have a soul at all. So using the dark side of one's soul in an aura attack wouldn't really transform one into a Grim would it?

As for Dust, yeah I think it either makes your particular Aura more potent or allows you to manipulate the energy your Aura can give you into other forms like fire, ice, and water.