Late in The Dark Knight Rises, Gotham is closed off from the rest of the world and the police are forced underground (both literally and figuratively). We catch up with the action when the government sends a highly-trained government agent in to help with the efforts to overthrow Bane, his mercenaries, and the criminals now free in Gotham. There are a number of times that John Blake is shown drawing a Batman symbol with chalk (shown below is an image of the album cover for the soundtrack that has a similar drawing on it).
What purpose do these drawings serve?
My best guess is that they had something to do with tracking the trucks carrying the bomb around, but that wasn't especially clear.