Yoru is considered the strongest sword in the world.
Even if Enma become a black blade, it will be either equal or slightly inferior to Yoru. For Yoru to be more impressive than Enma (well there's the iceberg mountain cutting feat from Mihawk in Marineford), I have 4 options:
1. Splitting the sky - This will establish both hype for Yoru and Enma.
2. Breaks another user's black blade - This would be great. Imagine if Mihawk's Yoru breaks one of Zoro's black blade?
But then it kinda goes against Mihawk's notion of black blades being unbreakable so I don't know about this one.
3. Have some kind of special drawback - If Yoru have the same haki exert effect like Enma but let's say Yoru exerts CoC haki instead of CoA, this would be insane feat for Mihawk for maintaining this. Of course Mihawk mastered Yoru and turned it into a black blade so it doesn't exert his haki anymore but that's my theory.
4. Showing better feats than cutting iceberg - Splitting the sky, splitting islands, splitting multiple mountains, breaks someone's black blade, have to master it through excellent haki control, all those can works.