Got curious about Black blades after seeing the Road to Laughtale Vol.2 which states only Haki is not enough to acquire a Black blade, at least it suggests it isn't.
With that said, I read Chapter 1023 which seemingly gives some explanation.
Zoro vs. King, Zoro said King wasn't a Swordsman.
King said he isn't, and he defined what a Swordsman is... a Swordsman is someone who sticks to a traditional style and form in battle in the TCB, whereas the Viz translation for King, he states he's willing to use anything to win a battle including biting his opponent.
In other words, a Swordsman is someone who will literally only rely on a Sword to win a fight, nothing else but Sword techniques.
I'd imagine Whitebeard and Roger weren't Swordsmen in the sense they utilise other methods to win battles, they're willing to win battles even without a Sword.
Black blades can probably only be developed be true Swordsmen, and I guess just like Enma was able to acknowledge Zoro and Oden, Swords can acknowledge true swordsmen which is part of the requirement to get a black blade.
Gyukimaru also said Ryuman's black blade was forged through many battles.
Adding everything up together, it seems a Black blade is formed by a Swordsman (someone who only relies on a Sword to win and is dedicated to Swordsmanship alone) through many battles. I guess you acquire a Black blade, which requires Swordsmanship during battles, extremely powerful Haki and acknowledgement from the Sword after many battles.