Base on what I know of the name, Muramasa, there is sight slight chance that the Bleach character Muramasa could have been the Zanpatutou of a older shinigami, who may have die long before the arc began. I believe this due to the fact that Muramasa was the name of a famous swordsmith from an earlier era of Japan. And the name, Muramasa, has been use many time in video games and anime, for a legendary katana. That just base on what I know. So yeah, we'll know more as the anime developed, especially since the arc was unexpected.