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 ...
Sorry, but the hand turned black from the ring.
p. 503
Yes indeed," said Dumbledore, and he raised his blackened, burned-looking hand. "The ring, Harry. Marvolo's ring. And a terrible curse there was upon it too. Had it not been--forgive me the lack of seemly modesty--for my own prodigious skill, and for Professor Snape's timely action when I returned to Hogwarts, desperately injured, I might not have lived to tell the tale. However, a withered hand does not seem an unreasonable exchange for a seventh of Voldemort's soul. The ring is no longer a Horcrux."