Given how hot it is depicted to be (i.e. blue flames) and the lighting around it, I also think it's plasma ^^.
It's important to note that the color isn't the only distinctive difference btw Ifrit & Diable, the lightning like strands are as well. One could argue it's due to Sanji using CoA but that particular lighting effect typically results from CoA clashes so it's not the same ting imo.
Will be interesting to see whether it's animated as crackles or electrical discharge ^^
Also for those of you than can be arsed to read it. This article gives a simple explanation on what a plasma is and how to determine if a flame is a plasma or not ^^
https://www.wtamu.edu/~cbaird/sq/2014/05/28/do-flames-contain-plasma/