In DragonVale for the iPhone and iPad, the Blue Fire Dragon is a hybrid dragon mixing fire and cold attributes.
To breed the Blue Fire dragon, you need to breed a Fire Dragon with a Cold hybrid.
Some combinations you can use however are Fire + Storm, Fire + Ice, etc. Fire + Storm seems to be one of the more popular combinations, since this combination also has a chance to produce the Sun Dragon or Moon Dragon.
Note that any of these combinations do not guarantee results, and the different combinations have different percentages for success. Keep trying though and you’ll eventually get your Blue Fire Dragon.