Masamune is Seph's Katana...
Masamune was one of Japan's fabled swordsmith's. He was indeed a Katana crafter. Murasame was another swordsmih, told to be Masamune's student. According to myth, Murasame challenged his 'master' to see who could make the finer sword. When the swords were done, they put them in a small creek. Murasame's blade cut through whatever it passed, while the Masamune cut nothing. A monk said that while Murasame's was a fine blade, it was evil and bloodthirsty. Masamune's was better, because it doesn't harm the innocent.
If you look at the Masamune as a sword of justice, then yes, Murasame would be it's evil counterpart. If not, then no.