EDIT: Since people are still answering this, the bug seems to have been caused by a health increase in the patch around that time and has since been fixed.
I know Blue regenerates. We took care of this by having each of us attack one leg, with the siren behind for rezzes. Also, I hit it with about 16 bonus packages (gunzerker). All three legs were bare for about five seconds, but Blue didn't go down, he still had about 1/16 of his health. It was late so we haven't popped back in for a rematch. All other playthroughs I've been in, Blue's been easy enough to kill before any regeneration can occur.
Is this a glitch, or is it possible to still beat Blue when this happens? We hit it with every element, however we could, and not a thing. As far as I know, they only take damage on the crystals, so I expected that to not work.
Edit: The lowest Blue's health went was the same each time, as we did break all the leg crystals multiple times. We started just meleeing, as Blue usually is easy with four people. I never saw Blue's health go back to full, now that I think of it, and we let it regenerate for a while at one point.
Best Answer
Blue's health was sort of recently increased with a patch; I suppose it's possible that introduced a glitch where he can have more health than destroying his crystals will reduce.
However in addition to his health restoring, the crystals on this legs will also regrow after a minute or two, adding back 30% of his health. Presumably waiting for his crystals to grow back will let you kill him.
I do believe it's a bug that you saw, he definitely can and is supposed to die when all three legs are "broken" and has died like that for me several times (there's even a challenge for killing him before his leg crystals respawn). But since his legs regrowing restores less than 1/3rd of his health I assume that by destroying them you'll eventually whittle him down but I have not personally seen him survive all three legs breaking. Like all crystalisks he doesn't have any weak points but his legs.