From my post 176
I would think that DD's hand turned black when he put on the ring (horcrux) because the ring started to control him and Snape had to kill the hand and destroy the ring to get it to stop. I would not want to be the one to drink from Hufflepuff's cup or put on the locket.
Good points. JKR is so good at laying out clues that it makes me want to go back and read all of the books again to see what I can pick up. Some time back I read an interview with her and she said that all the clues are there to figure out the ultimate outcome.
DD's "wand hand" turned black in book V when Voldemort aimed at his wand with the Avakadevra curse...and by DD's own admission he was growing weaker and weaker...this is why I think it was all a plan between Snape and DD...DD sacrificed himself ...