blind side (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]