Shoot the Messenger slows down Blighter lookouts after you kill fifteen of them before they sound the alarm. Lookouts run for reinforcements, so they are priority targets anyway.
Shoot the Messenger unlocks after this challenge: kill a lookout before he raises the alarm, 15 times. As with the other 28 base perks, the Shoot the Messenger challenge also counts when you play Lydia in the World War I simulation.