It makes no sense that GB has a black blade.
It is just another case of Oda intentionally baiting people to dig their own graves.
He didnt accidentally draw a katana without a scabbard, lol.
He wanted to create this argument and the OP fandom took the bait without blinking.
However, the crackhead has the urge to shit on everything and everyone.
I wouldn't be surprised if he gave it to Green Bull and then let Sanji beat him.
The ultimate troll move, too good not to do it.