User blog comment:DiegoAnime3000/Black is for Polarity/@comment-15145664-20140110234404/@comment-3257939-20140111002942

I knew I was missing something! Thanks for making me remember, of course, the DVD Commentary:

My friend Sorairo made a post about the things he heard in the Director Commentary, one of them was:

''Weiss' way of using Dust is more direct (Miles calls it "educated") while Ruby's way of using it is more simple. She "goes out and buys bullets that have different kinds of Dust in them" and then I heard something about the "gravity round" which might reference the "Cross Magazine" that we saw in the Red Trailer.''

Like you said, she switches to different bullets with a black muzzle flash, that must be the "Gravity Round", which is black color. The white flash changes into black and the bullets become stronger, much stronger than before thanks to the Cross Magazine she used which may have been full of Black Dust Bullets.