A blind jump is an operation in which a ship makes an FTL jump to a random location.
Blind jumps are usually done in the most dire of combat circumstances when a few seconds could mean preservation or destruction. In a blind jump, the navigation computer is bypassed and the jump system is most likely provided with a random set of coordinates with only sparse safeguards before the jump is executed. This greatly increases the chances of the ship getting itself hopelessly lost in space and adds to the risk of being in danger after the jump (impact with ships, planets or other bodies).
Fortunately, the vast magnitude of empty space in the universe makes the chance of such a collision relatively small.
Galactica herself executes a blind jump of sorts at the conclusion of the Battle of the Colony in order to escape the critically damaged Colony. The coordinates used were based on the notes of the Music and entered manually by Kara Thrace, miraculously leading to the discovery of an inhabitable planet.
Under normal circumstances, it is not an option for the Fleet, as ships would jump to different locations without any chance of reuniting.