it was in book 5. harrys blood is only important because it, along with the dream harry has in this book, is what got dumbledore to use his golden instrument (not the the penisive) to try adn find out and i belive the fact that the snake spilts into two shows that harry and voldemort hav two sepearte souls. I can see where your coming from about dumbledore not wanting harry to do anything rash upon him finding he was a horcrux but i belive that if dumbldore knew he would of told harry, espeically after the whole 'I am now going to tell you what i should of a long time ago' speech (book 4, part of speech on the blurb)