If Greenbull did create his own Black Blade, his special Nature logia powers may have facilitated it.. The fact that he can regenerate altogether from a sprout is the most interesting thing to me about his powers.. Not only that he can suck the life energy out of people which suggest he could share a part of his own soul to a Blade if it's a key requirement to the makings of a Black Blade..
Fujitora has a Supreme Grade Blade, it's not unthinkable that Aramaki could have Skillful Grade Black Blade, right under Ryuma.. A Great Grade Blade like Shusui would be pushing it maybe, but there's still a tiny slim possibility..
I think it's very likely that Ryokugyu did turn his Blade Black, and there are different Black Blade Grades.. Usopp got a Black Weapon too which everyone keep dismissing..
The Kuja arrows are permanently infused in base CoA, a single step away from Black Arrows..
Fujitora has a Supreme Grade Blade, it's not unthinkable that Aramaki could have Skillful Grade Black Blade, right under Ryuma.. A Great Grade Blade like Shusui would be pushing it maybe, but there's still a tiny slim possibility..
I think it's very likely that Ryokugyu did turn his Blade Black, and there are different Black Blade Grades.. Usopp got a Black Weapon too which everyone keep dismissing..
The Kuja arrows are permanently infused in base CoA, a single step away from Black Arrows..
And i agree people sleep on Ussop. They still don't believe he used CoC in Wano they think its BM.