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.