The disease was identified as Mycobacterial infection. There are several strains that infect fish, including bettas. It can be found in aquariums and aquatic environments in many regions of the world.
In this thread
(towards the end), two breeders, Coppermoon and Basement Bettas, write about sending their sick fish to a laboratory for analysis. The lab examined the fish and identified the illness as Mycobacteriosis. (See page 21 of the thread.)
Mycobacteria is common in aquariums. Some fish appear more susceptible to it than others. It does affect bettas of all colors. The issue of whether it affects blues more, or differently, is something that I haven't seen answered.
Here is a good article that disscusses Mycobacterial infections in fish: https://srac.tamu.edu/index.cfm/even...factsheet/231/
Also, Basement Bettas created a thread to discuss Mycobacteria:http://www.bettafish.com/showthread.php?t=125895