It's not unfounded. Do you think it's some sort of weird coincidence that a Black Blade looks almost exactly like a Sword imbued with Armament Haki?
Shusui is renowned for it's Hardness, Koka literally means "Hardening".
Black Blades are forged through battles.
Continuously pouring ones Haki into/hardening a blade during battles, until it eventually becomes permanently black makes complete sense.
But I'd love to hear your take on how one goes about forging a black blade that doesn't involve Koka.
So all swords absorb Haki?
Like you've stated , the only direct statement we have about Black blade is that one was forged through countless battles. We actually don't know the specifics of it.
The issue with claiming that haki is absorbed is that why didn't Roger , WB and co also have black blades? WB has even used his bisento longer than even Ryuma.
Also if swords absorb haki, then why wasn't Monet killed when Zoro cut her with Shusui?
I usually refrain from making such statements because the information we have is scant. I'd rather wait and see how Oda builds it.
Because right now, all we will have is theories which cannot be applied in an actual argument. It might end up being proved that your idea was right but right now, there's no way of knowing