Nope, there is major fault in this. Because by this Logic, Ryuma should have been capable of just walking around and picking anything up he wants, and turning it black. That's ridiculous, cause there is no indication in the manga whatsoever that something like that is possible. There is no "forging" involved in that, cause again, forging is not a one step process.
We are not told "Ryuma bloomed his haki to the point where he was able to turn Shusui Black."
We are explicitly told that through constant battle, Shusui itself was forged into a Black Blade. Nothing implies that the constant battles was increasing his Haki, and then his haki became so strong, he was finally able to just blacken the blade. If that's the case, then using Armament Haki, on an already Blackened Blade would be completely pointless. That means your Haki will never be strong enough to make a Blackened Blade harder than it already is. So why on Earth would Zoro be using Armament Haki on Shusui, if the blade is already too high of quality in comparison to his Armament Haki? It makes zero sense. You can't make something harder, that's already become so hard due to God Tier Haki.
Yes that is exactly what I am telling you, if Ryuma picked any blade he could have turned it into Black blade. Its makes much more sense than the Haki container bullshit that you are trying to pull.
What do you mean there is no indication of it, we know Haki blooms in extreme battles, we know Ryuma had countless battles, he took on the challenge of an entire world, probably from Admirals of his time, top pirates of his time, for fuck sakes he took on the World Government and World Government failed against him.
You are literally trying to reduce the weight of making a blade blade just because it doesn't fit your narrative.
Why would Zoro not coat Shushui with Haki when he himself did not know exactly how Black Blade is made. And maybe he did but just because CoA is what makes permanent black blade doesn't mean you get all properties of CoA, Ryuma made Shushui black centuries ago and main reason for coating your sword is so that the sword doesn't break.
But that doesn't mean permanent CoA would also include advanced CoA properties like shooting beams of Haki like that cut on Monet, the Haki tendrils that Zoro's and Roger's CoA produces, hell you can even cut an entire cliff of Wano with CoA, it was literally Enma shooting CoA out of Zoro's sword.
Even after you forget a black blade, have CoA strong enough to forge a black blade, all it would do is make the blade unbreakable, it is not gonna give you every single property of CoA. You can't shoot Haki out of a black blade without using external Haki lmao.