Has Zoro just unlocked black fire instead of Sanji?
It’s a new visual effect as far as I remember. There’s curling bits of it that remind me of CoA, especially the CoA we’ve seen around Luffy in G4 and Big Mom when she clashed with Luffy at the Tea Party. They have a more regukar pattern though, it looks planned, like Tattos’s, whereas Zoro’s looks almost like mist flowing around his sword. That might be because Luffy and BM were on skin, while Zoro is leaking out.
It’s not the same as what Roger and Whitebeard did. Their Haki was jet black, jagged instead of flowing and, most importantly, I’m 99% certain was actually CoC, not CoA. What they did was very much like a souped up version of the CoC clashes we’ve seen from Luffy vs Doflamingo and Luffy vs Katakuri, and it would make more sense for Oda to hype our first ever view of Roger vs Whitebeard as a CoC clash than a CoA one.
If it is possible for Haki to be put into flames, I think that’s what we’ve seen. Fire from Hiryu Kaen combined with a CoA overflow. If that’s not possible, I’m more inclined to believe it’s an overflow of Haki, because that’s Enma’s gimmick, and because it‘s just a bit too different to how Oda’s drawn fire before. Fire’s usually drawn with overlapping flames which this doesn’t have.