I know people want the CoC thing, but I'm on the team that Black Blades are the pinnacle of those who master CoA.
A CoA so well developed that even Adv. CoC pales in comparison. After all, CoA was always the "power haki", so I think it fits that it's strongest form is the strongest haki powerwise.
I would find it dope af.
But man...it would be a freaking hard troll from Oda, and could sympathize with the frustration, ngl.