Chapter 1023 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 and nothing else but Sword techniques.
I'd imagine Whitebeard and Roger weren't Swordsmen, they're willing to win battles even without a Sword. Black blades can probably only be developed by 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 once the Sword acknowledges you're the best Swordsman along with extraordinary Haki, you acquire a Black blade, that's what I think so far.
Not to say Roger isn't a Swordsman, but he's probably not dedicated to purely Swordsmanship the way Zoro or Mihawk are. Roger was probably an amazing Swordsman, but not a pure Swordsman if I had to guess.