How i see things:Enma emits excessive amounts of the user's haki.So it is a training tool for Zoro to Improve his haki mastery.Shushui>Enma because one is already a black blade and the other still needs to be forged into one!
Turning swords into black blades:each time the sword is embued with coa some of this coa strenghts the blade's composition,the final result of this is the complete blackneing of the blade!Enma does not use the coa that is slowly turning it into a black blade,it uses the users haki!As i see it Enma can turn into a black blade faster than other swords because of this unique property!I see Zoro turning Enma into a black blade in this fight,then recovering shushui and then turning Kuina's sword into a black blade!So when its time to fight Mihawk he has 3 black blades!
Kaido's reaction to Enma might prove this aspect of blades slowly getting strengthened with coa,he sensed Oden in the blade after all!