Blue is a special Crystalisk, found in Caustic Caverns, and is a part of the Safe and Sound optional mission. Players are tasked with retrieving a safe for Marcus in Sanctuary, which has been knocked down into the Caustic Caverns. When players come to the chest, it looks like a normal chest, but upon getting close, Blue will pop out of the ground with the chest on its back.
Blue is an extra large, blue-colored Crystalisk, whose attacks are very similar to the Volatile Crystalisk. However, they can also spit out Exploding Mini Crystalisks and it can regenerate its crystals on its legs if left unattacked for a short while. The best course of action in taking out Blue is to stay in the tunnel nearby and take shots at its legs from there.