The Black Blade aka Kokuto... What is it and what does it represent? This is my theory:
Gyukimaru told us that Shusui became black blade along Ryuma's many battles. While it is unclear what happens during many battles, we can assume one thing - Color of Armament gets improved more and more. Mihawk is said to have had many battles as well, according to his vivre card, he dueled strong swordsmen until he no longer had anyone left to duel.
Now, a lot of characters have a very high level of Color of Armament but why do they lack a black blade? Is their CoA weaker?
We should remember how Mihawk described himself - I am not a brute who hunts rabbits with a cannon. Another thing that Mihawk said was - Without subtlety, the sword is but an iron bar. I dont think that Kokutos are forged mid-battle.
While all the big names have large amounts of Color of Armament what they lack compared to Mihawk and Ryuma are control and precision, they are brutes after all. Now, the forging process of a black blade requires two components - Hardening and Flow, better said unparalleled levels of Hardening and impeccable control over the Flow.
While those levels of Hardening may be achievable for more individuals other than Ryuma and Mihawk, their control over the Flow probably fell short and is the reason why they never managed to forge a black blade. They probably tried but failed.
Many have noticed that Roger, Rayleigh and Shanks have used different swords during different times when we saw them in the manga.
Question is - Why? It is because they tried to forge a Kokuto and failed. Why would they change swords if they couldnt forge a Kokuto?
Because the failure of that process results in swords being shattered in pieces!
The process itself is the following - the Hardening needs to be so strong that it creates immense pressure on the blade, to the point that it starts combusting but such pressure eventually breaks the blade. Now, what prevents it from breaking is top-notch control over the Flow which "holds" the blade together by evenly distributing the pressure on all points of the blade. If that pressure distribution gets disrupted even a little, the result is the inevitable destruction of the blade.
The benefit of a black blade could be only what we already know - an object with incredible hardness. Basically, an invincible shield. Is there anything better that a swordsman could ask for?
That brings me to Zoro's attempt of forging a black blade in Wano. While his Hardening could have already achieved such levels that it could combust the blade, the control over the Flow is still not at required level but it took a step closer - that is what getting used to Enma is for, the control over the flow.
He will try to forge Enma into a Kokuto but he will fail and the blade will be shattered. Nidai Kitetsu will take its place after Wano, that blade has no other purpose than eventually ending up at Zoro's hip, nobody else wishes for a cursed blade.
So far, I think this theory works pretty well.
