@nik87 so you're saying that turning a blade black has to do with the flow?
Do you see it as a 'the flow is always "flowing" within the blade' kind of thing? As if the blade had a conscious of it's own of some sort...
I think I posted my theory somewhere, probably in Purpose of Zoro thread.
In short, the process of forging the Kokuto requires both Hardening and the Flow. Unparalleled levels of both.
While many top tiers may have Hardening at required level, they lack control over the Flow and precision compared to Ryuma and Mihawk since they are brutes, as Mihawk described himself as not being one of those brutes who hunt a rabbit with a cannon.
My theory is that Hardening reaches a level at which it creates immense pressure on the blade and starts combusting it but such process breaks the blade. What prevents it from breaking is the Flow which distributes the pressure evenly on all parts of the blade. Someone who read my theory said that it should be reversed and may be even a better way. It is flow, internal destruction which affects the properties of the blade and combusts it while Hardening is keeping it pressed so it doesnt break apart.
It was forged alongside many battles that Ryuma had. What happens during those many battles is that CoA gets better and better, it is related to growth of spirit(haki). It could also be linked to growth of spirit through mastering the 6 paths. Zoro's only new move post TS is called the Crossing of Six Paths and it could be a hint towards the ultimate growth of the spirit, the mastery over all 6 which signifies Haki levels at required mark for creating of a Kokuto.
Zoro has shown mastery over 4 paths so far and with Wano being hell themed, as well as Enma and the mastery of the same, it could mean that he also mastered the path of hell and in that case a new spirit manifestation could be expected that wasnt seen so far. In that case only a path of god would be left and works well for my theory which also says that Zoro will try to forge a Kokuto in Wano but will fail due to control over the Flow still not being at required level which aligns well with the lack of the mastery over the path of god.
Also, once created, the black blade only gains in hardness, I dont think it offers anything more than that. Could be wrong about that but an object of unparalleled hardness is an awesome thing by itself. Basically, an invincible shield. It is not hakified or anything of the sort, imo.