both black blades were introduced pretimeskip… we see Gandhi cleaning his non black blade in chapter 594 With his reflection in the dang blade. In chapter 599 oda has zoro cut the ship with shusui… if oda was gonna make ghandi have a permanent black blade he would have shown him with one back then when we saw a clear visual of the sword and his reflection in it. Oda isn’t gonna retcon anything there are two black blade introduced shusui, and yoru… nusujuro is not one of them.