User blog comment:DiegoAnime3000/Black is for Polarity/@comment-15145664-20140110234404/@comment-3146930-20140111023427

The problem is that the Black-Glyph has been seen for pushing and pulling. When Weiss loads Ruby into the sling-shot, she is actively pulling Ruby back getting ready to fire. Then in a later episode during her fight with the boarbatusk, the Glyph she is makes for her final attack turns black and she is launched at the Grimm. The Black-Glyph just seems to be a charged up version of the White-Glyph, as it only appears when Weiss needs to use a stronger version of her Push/Pull-Glyph.