Haki can neg swords, but they also can't neg swords
This is why we have statements about weapons being able to withstand their users tremendous haki, Like WB and Kaido
That means Supreme Grade blades for example can withstand the might of Prime WB and Rogers haki, whereas a lesser blade might eventually crack from it.
It's interesting because you use haki to reinforce your blade, but if you use too much haki and your haki is too strong, you could potentially break your weapon. I'm guessing from the internal pressure of using that much haki.
This being the case, because we know any blade can become a black blade, it shows you can't just blast it with super strong haki again and again if it's lower grade, because it would eventually shatter. So that wouldn't be how you create it one. You could get away with that with a supreme grade, but only 12 of those exist.
But once that lower grade becomes a black blade it's the most elite class and would definitely withstand that kind of pressure from haki.
So a black blade is not just some outer coating that makes it heavier and more durable, the actual make of the sword changes, maybe even on an atomic level. Which goes in line with "forging" it into a black blade.
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In terms of negating the "will" of a blade, I don't know how much that would help if roger is coating ace in his conquerors haki, he used his will to tame it, so the blade is acting in accordance with his will anyway. And you're dealing with his will when you deal with his haki. So you're just trying to negate Rogers haki at that point.