Instead of inventing tags like Dark_Alice for posts like post #365993, maybe we should just have a general tag indicating that there is an evil/dark version of a character in that image (assuming there isn't one already). Like alternate costume, but more specific.
Stuff as well established/canon as Dark Sakura would stay, of course; this would just add to that.
darksided? evil_costume_switch? evil_alternate_costume?
Updated