I'm pretty sure the two are the same. A Cross of Blades was one of my favorite so I really remember it and I know how you could get the two mixed up. When they end up underground its like the start of a whole different story in a way. In the beginning Kagome is sent to the Garden by her parents to become a Seed (I'm pretty sure that's what they're called anyway) and later on you find out its because she's not their daughter and not normal at all. She grows up there, meets Sesshoumaru, heads out on a mission with him, crashes, and ends up in the underground. The stuff she learns there leads to the eventual "drastic change, hair turning red instead of black and the destroyer of the world thing" that you were talking about. Hmm... me thinks I shall go re-read that now.
But does that sound right to you? Or have I jumped to a conclusion to early?