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Memory Alpha
Blue alert

Blue alert aboard the USS Enterprise-D

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, including, but not limited to, environmental hazards to the crew, main power failure, docking and separation maneuvers, and landing protocols, for ships with the capability.

The bridge lighting on the refitted USS Enterprise turned blue when it prepared to enter the Spacedock One in 2285. (Star Trek III: The Search for Spock)

The alert for starships on auxiliary power in 2287 was a blue status screen, along with the main lighting on the bridge switching to blue. (Star Trek V: The Final Frontier)

In 2367, the event of an imminent environmental systems failure or disruption resulted in blue alert being called in the affected areas in order to help affected personnel escape or safeguard their lives. In such a case, the Captain and first officer were each responsible for directing any crew present into the officers' designated turbolift, waiting by the door as the crew quickly filed in. Only after did the captain ensure that the first officer's turbolift was entered lastly by the XO, did the captain then enter their own turbolift, and was thus the last officer off the bridge. (TNG: "Brothers")

The blue lighting effect seen in "Brothers" was created in post production by altering the hue of the standard "red alert". In the remastered version of the episode, the lighting is incorrectly red once everyone has evacuated the bridge. However, it is present in the scene immediately beforehand, as well as in subsequent scenes in both engineering and on the bridge. Furthermore, the commentary track available for the episode on the Blu-ray release refers to it specifically as the only instance of "blue alert" on the series. [1]

The USS Defiant went to blue alert when engaging its Romulan cloaking device in 2371. (DS9: "The Search, Part I")

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. (VOY: "The 37's", "Demon") 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: "Dragon's Teeth")

Blue alert was also used when a Prometheus-class starship activated its multi-vector assault mode. (VOY: "Message in a Bottle")

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 The Next Generation.