blind spot n.

Old English blind "destitute of sight," also "dark, enveloped in darkness, obscure; unintelligent, lacking mental perception," probably from Proto-Germanic *blindaz "blind" (source also of Old Frisian, Old Saxon, Dutch and German blind, Old Norse blindr, Gothic blinds "blind"), perhaps, via notion of "to make cloudy, deceive," from an extended Germanic form of the PIE root *bhel- (1) "to shine, flash, burn."

The original sense would be not "sightless" but rather "confused," which perhaps underlies such phrases as blind alley (1580s; Chaucer's lanes blynde), which is older than the sense of "closed at one end" (1610s).

The meaning "not directed or controlled by reason" was in Old English. The meaning "without opening for admitting light or seeing through" is from c. 1600. In reference to acting without seeing or investigating first, by 1840; of aviators flying without instruments or without clear observation, from 1919.

The twilight, or rather the hour between the time when one can no longer see to read and the lighting of the candles, is commonly called blindman's holiday. [Grose, 1796]

c. 1200, "moral stain," probably from Old English splott "a spot, blot, patch (of land)," and partly from or related to Middle Dutch spotte "spot, speck." Other cognates are East Frisian spot "speck," North Frisian spot "speck, piece of ground," Old Norse spotti "small piece," Norwegian spot "spot, small piece of land." It is likely that some of these are borrowed from others, but the exact evolution now is impossible to trace.

Meaning "speck, stain" is from mid-14c. The sense of "particular place, small extent of space" is from c. 1300. Meaning "short interval in a broadcast for an advertisement or announcement" is from 1923. Preceded by a number (as in five-spot) it originally was a term for "prison sentence" of that many years (1901, American English slang). To put (someone) on the spot "place in a difficult situation" is from 1928. Colloquial phrase hit the spot "satisfy, be what is required" is from 1868. Spot check is attested by 1933. Adverbial phrase spot on "completely right" attested from 1920.