So the grand fire curses were basically fire cranked up to 11 in 5 different ways (scorch, inferno, etc.), and each having a special property and color, with aethereal fire being yellow, scorch being green, inferno being blue, promethean fire being white, and darkflame being black/purple/red.
Now, fire in real life can also be certain colors in accordance with temperature, with red/orange being the coolest, blue being slightly hotter, followed by purple (one of the hottest colors).
Green and yellow can’t be counted since they’re more or less because of added/subtracted chemicals in fire rather than the temperature or brightness of fire.
Now, as stated before, the temperature/color of fire is correlated, but also that every color emits a certain frequency or level of energy. Darkflame is mainly black or red; red has the lowest frequency of any color on the visible spectrum, and since black is literally just “I CANT SEEE” and isn’t light, Darkflame being naturally black would actually mean it’s incredibly cool for fire, and it being red would mean it’s the same temperature as regular fire. Then, Darkflame could just have a very cool temperature range.
obv this should be taken with a grain of salt as darkflame can also be purple (obviously to be more edgy) which would mean it’s not actually that cool at all (cause, yknow’, purple fire?)
conclusion: darkflame is cold af (i think)
Thank you for coming to my half-baked ted talk.