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Zoro already has a black blade...
And we already saw Titlehawk with a black blade that is supreme grade and it didn't help him kill the pre ts hakiless and badly wounded base Luffy.
Plus that a Marco underling without black blades or supreme grade swords forced the supreme grade black blade to flee.
If Sanji gets supreme king haki I don't know how Oda is going to justify Zoro being on the same level as Sanji.
And we already saw Titlehawk with a black blade that is supreme grade and it didn't help him kill the pre ts hakiless and badly wounded base Luffy.
Plus that a Marco underling without black blades or supreme grade swords forced the supreme grade black blade to flee.
If Sanji gets supreme king haki I don't know how Oda is going to justify Zoro being on the same level as Sanji.