No, that would give him a top tier logia + black blade + supreme blade. It would make him look an entire lvl above Fujitora on paper. Fujitora has been the most swordsman inspired Admiral and for Greenbull to not only have a black blade but also a supreme black blade would give Fuji nothing to stand out.
Great grade black blade vs Supreme Grade non black blade is a good balance.
If Greenbull doesn't have a black blade then it's fair game.
Same for me.
I have said in the past that I see Aramaki's situation like Ryuma's: O Wazamono forged black.
If he had a supreme balde too, he would be almost too much.
But very happy to be wrong in that case.
(GB still didn’t forged his black blade if it’s one doesn’t make any sense narratively if he did it himself)
In fact, It's the opposite that doesn't make much sense:
If he has one, it's because he forged it. If he's not a good swordsman, it doesn't even make sense for Oda give him a sword like that, let alone create a random samurai just for Greenbull to have stolen a sword that he doesn't even use properly.
Not to mention, can you Imagine that Oda, in the final battle against the Marine, creating a plot of a super random samurai having his sword stolen, just so a side character like Greenbull has a decorative sword.
And in all that you would STILL have a unnamed new character that forged a sword black. It has ten thousand times more sense It Is Aramaki in the first place.