I think it’s not a coincidence that the word Zoro highlights from this interaction is “forged”
See Mihawk more or less should have told Zoro how black blades are formed in the 2 year time skip but among all the things that Gyukimaru said, the thing got Zoro’s attention was the word “forged”
Meaning Mihawk didn’t mention this word when he was giving his overview of how Black blades are made.
If we take the word “forged” literally, it implies basically beating haki into the sword. Haki coating is exactly that, a coat of Haki around the sword. Whether CoC or CoA, the haki is applied around the sword. But forging implies the haki has to be infused into the sword itself.
That’s why Gyukimaru talks about Ryuma’s history of battle. Battles entail sword clashes. And sword clashes provide pressure for “forging”. I think Black blades are made when a swordsman infuses haki into the sword through the physical pressure of battles.
And no, this isn’t what Joyboy’s haki knot is. People misunderstand the haki knot. Joyboy said that he is going to “compress” a haki blast. Joyboy made a haki blast and then wraped it in rope knot. He didn’t infuse his haki into the literal molecules of the rope. He tied the rope around a haki bomb basically.
“Forging” a black blade would entail a haki ability we have not yet seen. Instead of CoC coating or CoA coating, it’s literally turning haki inside out so that it enters the molecules of an object and strengthens them from within


