I will go a step further..
Original Supreme Grade Blades '' Can not '' be turned into Black Blades.. Their Soul is too powerful to be turned into mere Black Blades..
That's why so many top top tiers have Supreme Grade Blades while not being Black..
Roger, Whitebeard, Shanks, Rocks, Nasujuro, Garling, Fujitora..
And now Imu.. A God Tier character, with God Grade Blade.. Nothing less would be a Slander to Imu's name, to consider the Sword of Nothingness as a Supreme Grade..
Again, Zoro needs to be chosen by an Original Supreme Grade Blade and be the first ever to 2nd forge that Blade into a God Grade Blade.. Else, Zoro will never be a Top Top Conqueror..
