I am sorry for your loss.... If this was the mystery disease (recently determined to be an aggressive strain of Mycobacteria), then there would have been nothing you could have done, even if you'd been home with him. The progression is extremely fast. Blue bettas seem to be particularly susceptible to it.
If you want to know more about this disease, there's a thread stickied at the top of the Diseases/Emergencies forum about it. Clicking this link should take you directly to page 61, which is where the disease was identified as being Mycobacteria: http://www.bettafish.com/showthread.php?t=90086&page=61
Again, I am sorry for your loss.