I think it is going to be Tom Riddle, a.k.a Voldemort. First of all he is half-blood. Secondly, even though JK specifically said Voldemort wouldn't be the half blood prince, I think she said it like that for a reason. When Dobby warned Harry about who was doing the attacks in the second book, Harry asked him if it was Voldemort, but Dobby said it wasn't Voldemort trying to give him a clue that it was not He-Who-Must-Not-Be-Named, it was Tom Riddle, Voldemort's former self. So I think JK is using the same tactic as Dobby, telling us it is not Voldemort, giving us a clue to think back to the previous books