OK, about the X-Blade (which, for those who don't know, is pronounced "khi-blade." Meaning no different than "keyblade." Anywho...), it wasn't the point of the Keyblade Wars EXACTLY. People were trying to get it, yes, but ultimately the War was for Kingdom Hearts. They just needed the X-Blade to unlock Kingdom Hearts. It is made from a perfect balance of Light and Darkness, and is therefore both and yet neither.
As for the DtD (Door to Darkness) that was not Kingdom Hearts. That is just the passageway between the Realm of Light and Darkness. Kingdom Hearts is in the Realm of Darkness.
So to answer the main question of this thread, no, Kingdom Hearts is not a Heartless because it chose by itself to essentially HIDE in the Realm of Darkness so that no one could get to it and abuse its power. Therefore, it is not a Heartless. Darkness did not consume Kingdom Hearts.
EDIT: Oh, and about "X-Blade meaning 'death blade'" thing, it's another pun on the translation. X, or "chi" in Greek can also be translated as "power" or "death," so that's where that came from.