User blog comment:Kajune/Does Dean require demon blood for Michael to possess him?/@comment-3481504-20151229202330/@comment-3481504-20151230183956

No, I'm not saying his form was hurt, but it could've changed in some way. My guess is that "falling from grace" and personally molding a human soul into a demon might've taken such a toll on him that it altered his form in some way. Now there was absolutely no explanation for why Lucifer needed demon blood to sustain a form and Michael didn't, but my guess is as good as any.