Connelly titles the novel after a nickname that Bosch and the tunnel rats gave the tunnels during their time in Vietnam. It was first coined by Meadows upon emerging from a tunnel with grisly war trophies, and it is defined in the novel as “the darkness, the damp emptiness you’d feel when you were down there alone” (318). The black echo describes a feeling of being dead and alive at the same time, of being scared that your own breath will give away your position to the enemy; Connelly elsewhere describes the terror evoked by the dark entrance to a tunnel with a reference to Edvard Munch’s painting The Scream. This horror haunts Bosch’s memories even 20 years later.

In Los Angeles, the darkness of tunnels also symbolizes the underworld’s “ugly story” (59)—a criminal hierarchy where an underclass of people like Sharkey, Arson, and Mojo are trodden on by more powerful evildoers like Rourke and where the veterans who sacrificed their youth in the Vietnam War turn to substance use and violence when they cannot reintegrate into civilian life. The implication is that anyone forced to confront the disorientation and terror of the black echo might be corrupted by its malevolent aftereffects, going on killing sprees like Meadows or being seduced by get-rich-quick schemes like Wish’s brother.