A Blue Fire dragon is a hybrid dragon made of opposing elements. The general rules on how you can breed them can be found at this question.
Specifically for a Blue Fire dragon you will want to select a Fire Dragon and then any other Cold Hybrid dragon. At this time the list of cold hybrids are:
- Blue Fire
I have not put the Frost Fire dragon in the list as I am not sure that will get you a blue fire dragon since they are somewhat opposites of each other (You do a Cold dragon + a Fire Hybrid to get a frostfire for example). I also left off the Sun or Moon dragons as they are rare dragons but they should, in theory, also be able to be used.
Hope this helps.