Don't you see a Pattern here?
Luffy vs Kaidou is like two faces of the same coin
Luffy have Sun God DF while Kaidou have Dragon DF
I don't think it's coincidence that Shanks is hyped as greatest CoC User while Mihawk's Name "Dracule" means Dragon
And Shanks is one who brought DF to where Luffy was, while Mihawk is one who trained Zoro
Luffy had his Eye Scar when he was with Shanks, while Zoro had his when he was with Mihawk
... etc
I believe Shanks gained his Unique CoC from defeating a "God" or some kind of interaction with one
While Mihawk made his Sword Black by slaying a "Dragon" or some kind of interaction with one
I think the reason Mihawk have Black Blade while Roger, Oden, Rayleigh, Shanks, WB ... etc don't is because it requires a Dragon Sacrifice.
You don't think it's a coincidence too that the only other person who had a Black Sword (Ryuma) is known for slaying a Dragon (And even though Onimaru said that Ryuma have many Legends, Oda decided to only highlight this one)
I'm pretty confident that Black Sword is not earned by Mastery but rather by a specific Condition
Zoro is gonna turn his Sword Black by slaying Kaidou (However it's possible this might not be the end, cuz Kaidou's DF gonna trigger something too)