A communications blackout was a term used to describe a total stop of all incoming and outgoing communications.
In 2268, the USS Enterprise maintained a communications blackout while on a top-secret diplomatic mission. (TOS: "Elaan of Troyius")
In 2369, the USS Enterprise-D's stellar cartography department requested a communications blackout while they performed an experiment. (TNG: "Lessons")
In 2374, Kira Nerys ordered the USS Defiant to maintain a communications blackout as the ship confronted a Dominion fleet. (DS9: "By Inferno's Light")
In 2374, Ferenginar had a communications blackout, convincing Rom that the planet had been conquered by the Dominion. (DS9: "Profit and Lace")
In 2375, Julian Bashir was unable to contact Deep Space 9 while attending a medical conference held on Romulus because of an imposed communications blackout. (DS9: "Inter Arma Enim Silent Leges")