Dark Magic eh? My bet is that it's a transfiguration of some sort, as Ron's real head would never have that trace (more a beacon, really) of Dark Magic, because then Draco's fingers would have the same trace, as they seem to be amputated in the same way.

Very creepy still, though Harry seems to be inclined to think in terms of gray where Draco is concearned, but about his sexuality.