I was thinking about Tenguyama Hitetsu’s comment that Zoro might be capable of making Enma a black blade.
Zoro can already extend his Haki in battle so his swords are protected. But they don’t stay black. Something needs to occur to make them black. I think that something is Conqueror’s Haki.
Several times I have seen Zoro display what I have interpreted as Conqueror’s Haki. He gets a really serious look on his face, his pupils dilate surrounded by a small gray circle (which Oda does a close up of), and he just glares at someone. That person immediately backs off… usually saying something about the look in his eyes. I don’t think this is merely intimidation. I think this is his Conqueror’s Haki and it just hasn’t fully awakened yet. I think for a Swordsman to make a black blade, he has to extend his will against the swords to be worthy, so to speak.
We have already seen Zoro assert his will over his swords with both the Sandai Kitetsu and Enma.
I think he needs to use Conqueror’s Haki though to truly command them. Once he does this, he can make both Enma and Wado Ichimonji black blades.
*Theory by Dark_Sidhe