In the final season of Angel, Fred was infected by some ancient dust which caused her to become possessed by an evil God named Illyria. Upon transforming into this (highly annoying) Being, her hair was streaked with blue. It was as if the dye had run down her forehead but her incompetent colorist failed to wipe it off.
I was reminded of this while browsing JustJared a moment ago. At least Evan Rachel Wood's hair isn't blue.