Well, not our "fault".
Ask Oda why he has drawn a thing that looks exactly like a Black Blade in the hands of a top tier who in Roningai, the film-inspiration for the present trio of admirals, was a swordsman.
All the rest is natural.
There are two known black blade wielders in this story, one is the world's strongest swordsman, the other was known as the God of blade. Greenbull has zero hype as a swordsman. In fact, its not proven that he is a swordsman.
Black blades are forged through a history of countless battles. Greenbull hasnt fought a single time with that sword. Even when he got serious against Momo, he still relied on his devil fruits to skewer Momo. A yonko level samurai with AdCoC and VOAT, Oden, who lived and died by his sword couldnt forge a blackblade throughout his life. But Greenbull, who rarely fights with a sword, has forged a black blade? I know you admiral fanboys are not the sharpest tool in the shed, but this shouldnt be so hard to grasp