Blue Alert is an alert status on Starfleet vessels with landing capabilities. When a starship goes to Blue Alert, it is 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 / lift off procedures. The Intrepid class starships such as the USS Voyager has landing capability. (VGR: "The 37's").
Blue Alert is also used when a Prometheus class starship activates its multi-vector assault mode. (VGR: "Message in a Bottle").