Um, this may sound stupid. But, you know in BBS when the Neo-Shadows attacked Ventus, Master Xehanort released his darkness, and Vanitas was created, how could Heartless exist back then? My thought on the Heartless, well the Pureblood Heartless origin, anyways. It involves the Keyblade Wars. I believe the Keyblade Wars that Master Xehanort spoke about were the creation of the Heartless. I think that someone was able to create an X-blade and released the power of Kingdom Hearts. But, something happened and Kingdom Hearts turned on all the Keybladers and turned them into Heartless. I think the only Heartless that were around were the Keybladers' Heartless. And, that they didn't increase until the creation of the Emblem Heartless and the worlds fell to darkness.