Monsters manga? It didn’t look black at the end of it.
And why wouldn’t the PH dragon make a sword black?
Even if it didn't turn Black, do you think Ryuma attacked Cyrano or The Dragon without using CoA at least?
We all know that before Post-TS, Oda never showed Haki Effects (Blackness & CoC lightning & stuff like that)
So the same thing applies for Monsters, the Story is part of One Piece Lore, but what's drawn doesn't reflect One Piece World
Haki wasn't demonstrated nor was the Sword Used looked like Shusui nor was Wano mentioned ... etc
But it stands that in Manga, it's said that Ryuma have many Legends, but only the Dragon Slaying one is always highlighted.
I already answered the question for Punk Hazard Dragon above, and personally i strongly believe that The Full Moon is another requirement
Some or all of these requirements are needed to forge a Black Sword:
1. CoC
2. Fulfilling The Sword's Purpose (Which is to Kill & in Enma's Case, to take revenge on Kaidou)
3. Slaying a Dragon
4. Full Moon Night
Think of it this way:
"When a Swordsman fully masters a Sword & uses it to Conquer a Dragon under Light of Full Moon, a Black Sword will be forged"
Listen, if you wanna know the entire Story of Sun God Nika, Story of Dragons, Role of Zoro & Black Swords & importance of Full Moon.
Simply re-read Drum Island Arc (Chopper's Arc). The foreshadowing there is insane