Board Thread:Announcements/@comment-2110162-20150722203346/@comment-3542404-20150722211221

NOormally flame colour depends on what gasses a being burned, blue indicates that something like butane and a lot of oxygen is being consumed, which produces a hotter flame (ie the colour is a by-product).

If you want to call out coloured flames silly, then I should point out that Azula's blue Firebending is technically not possible through Firebending alone as she can't control the specific makeup of the air around her. Assuming that that's how the colour is being changed and it's not something else like chi-related (which is an integral part of all bending).

Therefore, if it is governed by something other than the chemical makeup of the air, then there's no defined reason as to why it can't have evolved into the Enhanced Fire skill.