I like the idea of Sanji's final power up resulting in black flames, but that won't happen for a while, and I already don't like the frequent usage of Ifrit.
Diable Jambe is supposed to be absurdly hot as it is.
A kick so hot that it makes a master of Tekkai feel powerless, and as though his very bones are on fire.
Science and the rules of physics have their place in One Piece. Diable Jambe should still be viable for regular use, and Ifrit Jambe should basically be so dangerously hot Sanji can't use it freely without passively causing unintended environmental damage and making allies pass out from the heat.
If Sanji does get black flames, it should coincide with CoC. I'd like to see what a CoC infused kick looks like.