In my opinion, to make a sword permanently black, you must fulfill the sword's wish, it's not enough to be powerful, nor did Oden or Roger turn their blades black. Zoro IMO until the fight against Mihawk will only be able to fulfill the wish of Wado Ichimonji and perhaps Sandai Kitetsu (because she is weaker than her sisters and wielded by the WSS). Enma IMO's wish is to slash a dragon like Kaido, something Zoro will only do in official novels many years after the end of the main story of the work.