Considering the near mythical status Ryuma had in the world, and how it is literally the only other black blade aside from Mihawks Yoru we know, it just seems weird to me that Oda would not just make it a Supreme grade sword too. Especially considering we only know 3 of the 12 it is a bit confusing to me to not make the only other known black blade from a character literally called the Sword God into one of these 12 sword too.