Is currently hard to find a way in which classic dragons could develop and use the fire-breathing, so find a way in which this could happen for blue fire of complete combustion is even harder. Read this How could dragons be explained without magic?
I was wondering this because blue fire have some important advantages, is hotter, is more efficient, more controllable (it does not expand so quickly and chaotically) and therefore precise, it also does not produce ash, soot or carbon monoxide, besides it can be said that it is capable of "burning" normal fire.
The notorious problems for blue fire breather dragon is that blue fire is hotter and if it's hard to deal with the normal fire temperature this is even harder, and the other problem is the raw material, I don't know if a biological system is able to produce or get in a relative easy way the required composals, a good material for complete combustion is the n-heptane.
And finally in an evolutive vision, why would a fire-breathing dragon need this fire?, if normal fire is enough.