"partial blackout," 1942, based on blackout in the "dousing of lights as an air raid precaution" sense; from brown (adj.) as "not quite black."
Entries linking to brown-out
blackout n.
also black-out, 1908 in the theatrical sense of a darkened stage, from the verbal phrase; see black (v.) + out (adv.). The figurative sense of "loss of memory" is by 1934 (verb and noun); as "a dousing of lights as an air raid precaution," it is recorded from 1935. The verbal phrase black out, in reference to using black ink to cover printed or written matter deemed objectionable, is attested from 1888.
brown adj.
Old English brun "dark, dusky," developing a definite color sense from 13c., from Proto-Germanic *brunaz (source also of Old Norse brunn, Danish brun, Old Frisian and Old High German brun, Dutch bruin, German braun), from PIE root *bher- (2) "bright; brown."
The Old English word also had a sense of "brightness, shining," preserved only in burnish. The Germanic word was adopted into Romanic (Middle Latin brunus, Italian and Spanish bruno, French brun).
Brown sugar is from 1704. Brown Bess, slang name for old British Army flintlock musket, is recorded by 1785. Brown study "state of mental abstraction or meditation" is from 1530s; OED says the notion is "gloomy." Brown-paper "kind of coarse, stout, unbleached paper used for wrapping" is from 1650s.