Could it be possible that the Obliivion keyblade be the one Riku used when Ansem was in control of him.Couldnt be it since Riku always said "Welcome Oblivion" when you fight. It could've changed when Sora turned into a heartless, well if it's part of a Kh2 spoiler dont tell since when Sora uses his Drive's he either has Kingdom,Kingdom Oathkeeper, Kingdom Oblivion,Oblivion Oathkeeper keyblades. Could be like he always he carries something from Riku and Kairi.