achieving the blacken blade trait isnt just a matter of hacky, strength or experience (Oden,Shanks coudnt) but is relate to something much more spiritual.
1033 said that each blade have basically a personality a will, each blade choose the owner.
Why shanks and co didnt get a blacken blade? Well because they didnt explore completely the will of the blade,
Oda decided that Enma his the main blade in Wano, he is dedicating a lot chapters for this sword which is weird because Oden used 2 blades. Oda also decided that the blacken blade narrative was introduced when Zoro was fighting the fox(monk) during the battle of the bridge. After that Oda also decided to even reinforce the concept of the blacken blade with Zoro, Hitetsu was certain that Zoro could make Enma black. So how can Zoro achieve that? Well 1033 was a big step, to get a blacken blade you have to understand what Enma will is and what the blade wants. Zoro has reach the first step which is to give what the blade wants now he need to understand what the blade will is. Kozarubo said that blade are for killing and Enma was the best blade he ever crafted.
For Enma call the king of the underworld what is the will behind this blade? Well if blade are up for killing, probably Enma want to get the biggest price that there is. And what do we get in Wano? Well the so call strongest creature alive in One Piece.
The previous owner failed in taking kaido head that why he couldn't get Enma black but the new one is up for the task.
If Zoro cant achieve this trait in Wano, then Hitetsu moment was useless because Oda already mention this aspect during the fight of the bridge. So there has to be a pay off in Wano.
@The White Crane @Den_Den_Mushi what do you think about this theory?