Azula’s fire was blue because she was significantly skilled and powerful as a firebender, thus making her flames hotter and more intense. Read more here: http://avatar.wikia.com/wiki/Azula
I am assuming you are referring to Legend of Korra. :) LOK is set 70 years after Avatar: The Last Airbender, and it features the Avatar that follows Aang: Korra.
You can start watching Legend of Korra on iTunes here: