It'll likely be related to how much a person has fought with the said sword.
However, if Zoro turning Enma into a black blade will be an example of how other swords were turned into black blades... then It'll likely be a multi-generation process. Oden used it his whole life, pouring Haki... now Zoro will be using it.
Wouldn't be surprised if all supreme blades have their own "passed down" legends.