Blue alert aboard the USS Enterprise-D
Blue alert (also code blue or condition blue) is an alert signal status on Starfleet starships and outposts which is called for in exceptional situations, ranging from environmental hazards to the crew, to docking and separation maneuvers and landing protocols, for ships with the capability.
In the event of an imminent environmental systems failure or disruption, blue alert is called in order to help affected personnel escape or safeguard their lives. (TNG: "Brothers")
On a vessel with landing capabilities, such as the Intrepid class, blue alert is called as an indication that the ship is preparing either to land or to take off; the crew would then need to report to code blue stations, in order to aid in the landing/liftoff procedures. (VOY: "The 37's")
Blue alert is also used when a Prometheus-class starship activates its multi-vector assault mode. (VOY: "Message in a Bottle")
The bridge lighting on the refitted USS Enterprise turned blue when it prepared to enter Spacedock. (Star Trek III: The Search for Spock)
The USS Defiant went to blue alert in the event of engaging its cloaking device. (DS9: "The Search, Part I", et al.)
- This could also indicate that blue alert would be called on Galaxy-class ships prior to saucer separation, although this protocol was never referenced in any of the instances of that occurring on TNG.