I would love if Zoro turned Enma black and not only as soon as it's feasible. I would love to find out what it takes first of all. If it's an ACoA or ACoA + ACoC feature or even something different. Haki doesn't seem to be stored in blades and perhaps hardness is the characteristic that could be augmented. But overall the feat itself would be magistral as Shanks, Roger, Whitebeard seemingly didn't manage rather than actually rejecting the offchance to render their blades black, most likely.
One of One Piece's MAJOR theme is "Inherited Will". So What is "Will" Its Haki and ones goals and wishes.
So When you mention haki doesn't Store itself into blades like a power battery storage I agree; It doesn't store CoA into the blade from its previous owners
but it does carry his Haki "Will"(Desire and Wishes).
Like Oden's "Haki" (Will, Desire, Wishes) through Enma to Slay Kaido or Kuina's through Wado Ichimonji to become the Greatest Swordman.
So when it comes to making a Black Blade why have most of the powerful people not been successful into making one. The reason i explained in my other post is because i think its most likely due to People like Oden, Mihawk, Ryuma and Kuina carry their Haki "desire and wishes" into their blades because their "Haki" wishes and desire are directly connected to their swords.
Ryuma became a Sword God,King = Black Blade = Will fulfilled
Mihawk became WSM= Black Blade = Will fulfilled
Oden Failed his Greatest Desire to Slay Kaido thus unable to make Enma Black. Enma still Carries the "Will or Haki" of Oden to Slay Kaido which would turn it black when fulfilled.
whereas people like Roger, WB and Shanks "Haki" greatest wishes and desires are not connected through their blades. Roger and Shanks case maybe its the Strawhat but they don't fight with it so.....