I was thinking about Tenguyama Hitetsu’s comment that Zoro might be capable of making Enma a black blade.
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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.
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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.
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We have already seen Zoro assert his will over his swords with both the Sandai Kitetsu and Enma.
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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