In Avatar: The Last Airbender, Azula is the only character whose fire is shown to be blue. Is there any other character capable of producing or seen using blue fire?
In Avatar: The Last Airbender, dragons are shown to be capable of producing flames of many colors, including purple/blue.
However, no human characters, other than Azula, are ever shown producing blue fire.
No, there have been no other characters in the Avatar universe shown to have blue fire.
To elaborate, Azula's blue flame is due both to her heritage and to her single minded desire to be a worthy successor. One can improve one's bending, and one can be naturally gifted, but it was Azula's singled minded pursuit of perfect that caused her flames to be blue.
Related: Why is Azula's fire blue?