Here is the thing though, When Zoro first started fighting King, there was no flame around his sword, despite the fact that he was using CoA. It wasn't until Enma started acting up, that the smoke appeared around Zoro's sword. Zoro then was trying to figure out, how to stabilize what Enma was doing, so it wouldn't kill him. Once Zoro stabilized it, that's when King made the comment about Zoro wanting to become a King.
Yes, Black Haki lightning also started pouring out at that time, but we've already seen that happen on different occasions with only CoA. The permanent smoke around Zoro's Sword however, ONLY happened once Zoro stabilized Enma, and King made the comment about Zoro becoming a King.