From “http://flashforward.wikia.com/wiki/Blue_hand” :

A blue hand is one of the clues in the Mosaic Investigation; it turns out to be the symbol of Already Ghosts, an organization of ghosts who gather at Blue Hand clubs. A blue hand is also the title image of episode 1.06 “Scary Monsters and Super Creeps”.

“BLUE HAND” was seen in episode 1.01 “No More Good Days” as one of the Mosaic Investigation post-it notes.

In episode 1.06 “Scary Monsters and Super Creeps”, a blue hand is found on the body of one of the assassins from the previous episode, 1.05 “Gimme Some Truth”. Based on Mark Benford’s flashforward clue that a blue hand was linked to Baltimore, Demetri Noh decides to investigate Baltimore St. and brings Al Gough along with him. They find blue hands pointing the way to a Blue Hand club, where they find dead bodies with blue hands.

Al, Demetri and Mark later visit a Blue Hand club and arrest a member, preventing him from committing suicide. (“The Gift”)