Blue is an unusually large crystalisk encountered in the Caustic Caverns. Blue exhibits many of the same behavioral patterns as its smaller counterparts. Its ranged attacks include shooting several fast-moving volleys of spikes or a single large projectile in an arch towards the target. While the first is simple to evade and does little damage, the second will explode a short time after its contact with the ground, causing a large amount of explosion damage in a fairly wide radius. Blue's melee attacks are also similar to other crystalisks, which include stomping the ground with its forward-facing foot and causing AOE (area of effect) damage, or striking the ground with the center of its body, also causing AOE damage and knocking nearby enemies backward.
Blue is mentioned in ECHO recordings as having been found and befriended by Dahl security officer Elyse Booth. When Overseer Harchek ordered that crystals be harvested from the crystalisks, the results proved fatal to the previously docile beasts and those remaining, with Blue among them, retaliated. Harchek's final recording indicates that it was Blue that killed her personally.
- Main article: Safe and Sound
- Characters standing underneath Blue when it is killed will be trapped by its body. Once Blue's legs are buried, they will be able to move freely.
- Killing Blue before any of his leg crystals grow back completes the 'Harchek's Revenge' challenge.
- Blue has a higher chance to drop the Fabled Tortoise shield when defeated.
- As of Patch 1.2.0, Blue has increased health.