[Spoilers] Theory about the Black Swords

Simply put, I think the Black blades of OP have been forged with Armament Haki infused in them, making them far stronger than conventional forging methods.

When forging a sword, there a portion of the process where the smith dips the blade in a liquid, called Quenching, where they basically use carbon to fill the imperfections of the blade, making it heavier and harder. I believe that master smiths use a technique where they do this with their Haki, infusing it with the strength of their willpower.

We've seen what haki does when use on a normal fist, it literally hardens it. When used on a blade, it has the same effect. It would make sense that somewhere along the line someone thought to use armament haki when forging a blade.

We've already seen Zoro comment on Shusui being heavier than his other blades, which lends a bit of support to my theory, since conventional doping makes blades heavier and stronger, it would make sense that Haki doping would likely double that.

Obviously the main point against this is that if the blades forged with haki, they should be able to damage devil fruit altered physiques, like logia users, and Buggy. The only thing I have to say against that is that perhaps once the haki is no longer coming directly coming from the user it loses that aspect, and only functions as a hardening method.

If you guys have anything to add/contribute, I'd love to hear it, as well as any flaws/critiques! Thanks!

EDIT: u/corpseflakes comment got me thinking about haki. Specifically the Kuja warriors and how they infused their weapons with armament haki, yet they didn't blacken. Now we already know there's levels of observation haki, and conqueror's haki, so there must be levels of armament haki. I think that there must be a beginner level where it doesn't physically manifest, which the Kuja warriors use.

Now, I think this must play into why graded weapons are as strong as they are. If they were forged through normal means they might be great weapons not necessarily legendary. However, we've also seen the level that it takes to be legendary (Whitebeard, Roger, even the Strawhats in their own rights), and just being exceptional is not enough. Everyone we've seen that has been a match for Luffy has had some haki. Look at Sabo, all he had in the beginning of the tournament was haki, and he beat everyone in the ring with only that strength.

My opinion is that each of the Wazamono blades was forged with some level of haki, ranging from beginner's level, to that of an expert, and that is what sets them apart from regular blades.

TL:DR- All the graded/legendary blades of One Piece have been forged with some level of armament haki, which hardens them, and increases their overall strength, tipping them from being a well made sword, to being a Legendary weapon. Black Swords have been forged with top tier armament haki, which may put them above non-black blades.

Thanks for reading!