Flames are an indicator of Enma sucking up haki. Zoro trained to control his armament haki, and in his fight against King flames only reappeared when Enma sought conqueror haki. It's not that hard to understand. Right now Enma is demanding conqueror's haki, and you can see the same black lightning he had during King's fight. What's the problem? This is not a Zoro downplay, but a Lucci feat. He's probably stronger than King.