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Extinguish all lights. For example, The whole town was asleep, as blacked out as London during the war . In the early 1900s this expression alluded to the lights in a theater, but from about 1940 on it meant darkening an entire city to hide it from enemy bombers.

Obliterate with black, as in crossing out words on a page or print on a screen. For example, They have blacked out all the obscene words in the subtitles to make this movie suitable for youngsters . This usage may be derived from an earlier meaning, “to stain or defame,” which dates from the 15th century (and probably alludes to “blackening” a person's reputation). [Mid-1800s]

Lose consciousness, faint; also, experience a temporary loss of memory. For example, I couldn't remember a single note of the music; I blacked out completely , or The accused man claims he blacked out after his first drink . This usage is thought to have originated with pilots, who sometimes fainted briefly when pulling out of a power dive. It soon was transferred to other losses of consciousness or memory. [c. 1940]