Capt_mermain1
I don't think that's how 12 supreme grade swords works. I'm pretty sure they are 12 Supreme grade swords with or without the user. They also can't raise in levels(officially) just by being a black blade. They can however, raise in level(practically) like how shusui is at the level of 12 Supreme grade swords despite being among the 21. Also, along with Yoru, 12 great grade swords where made to be that way and Mihawk, being the greatest swordman skill wise and however black blade works, made Yoru the strongest sword so far.