Blue alert (also code blue or condition blue) was an alert signal status on Starfleet vessels and outposts which was 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 was 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 was called as an indication that the ship was 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. Blue alert was the minimum required ship status for landing an Intrepid-class starship, as the USS Voyager did make a landing while at red alert without switching to blue alert status. (VOY: "The 37's", "Demon", "Dragon's Teeth")

Blue alert was also used when a Prometheus-class starship activated 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 when engaging its cloaking device. (DS9: "The Search, Part I")