Blue Alert is a type of alert status with a variety of meanings, varying from starship or outpost to starship or outpost. Also known as Code Blue or Condition Blue, this alert can be used in a variety of situations and launches a variety of Starfleet procedures and protocol: on some ships, it signifies an environmental threat to the crew; on other ships, docking maneuvers; and on yet other ships, it shows landing procedures are under way.
Code Blue was issued as an alert for "statements to be made by yeomen Burke and Samuel" as part of a ruse to trap Lt Levarus in the Klingon/Federation attempt to undermine the peace treaties between both empires. (Star Trek VI: The Undiscovered Country)
Some ships, like the Prometheus-class, use blue alert to indicate a separation sequence into multi-vector assault mode. (VOY: "Message in a Bottle") It remains unclear if ships capable of saucer separation like the Galaxy-class also go to blue alert.