Didn't Mihawk say that ANY BLADE can become a black blade?
By "any", it should mean EVERY BLADE.
Some trash ass kitchen knife can become a fucking black blade.
Black blade is about the swordsman haki. Not the blade itself.
Swords have degrees of quality based on the materials used and the swordsmith skill. It makes it more durable, or gives it a sharper edge.
In One Piece, we have the sword ranks or whatever. They are way more durable than your average sword.
But we have Haki now. Mihawk said that, if you can control your haki well enough, your blade will never break and told Zoro to learn how to turn his blades into black ones.
Swords in One Piece world were never Zanpakutos. They were always just swords. As swords in real life, they can vary in quality, but they never had this "inside power" BS. They had "quirks". Sandai had bloodthirst of her own, which Zoro learned how to handle, Shuusui absorbed the others slashes to create a unique more powerful one, instead of 3 weaker ones. Zoro learned how to handle it as well.
Enma literally absorbs Haki, which is AGAIN the "control the haki flow" training Zoro ALREADY had with Mihawk.
But now it's the reverse. Before Zoro was training so that his blades could handle his attacks, by imbuing his haki on them, otherwise they would break. Now Zoro is playing catchup with his new sword. And have to "draw more power from the blade".
It takes away merit from his own power, to give to the sword power.
They way things stand >>>>>>>NOW<<<<<<, Zoro can only cut Kaido if he uses Enma's power, not his own.