late 14c., bluschen, blischen, "to shine brightly; to look, gaze, stare," probably from Old English blyscan "blush, become red, glow" (glossing Latin rutilare), akin to blyse "torch," from Proto-Germanic *blisk- "to shine, burn," which also yielded words in Low German (Dutch blozen "to blush") and Scandinavian (Danish blusse "to blaze; to blush"); ultimately from PIE *bhel- (1) "to shine, flash, burn."
For vowel evolution, see bury. Sense of "turn red in the face" (from shame, modesty, confusion, etc.) is from c. 1400. Related: Blushed; blushing.
mid-14c., "a look, a glance" (sense preserved in at first blush "at first glance"), also "a gleam, a gleaming" (late 14c.), from blush (v.). As "a reddening of the face" from 1590s. Meaning "a rosy color" is also from 1590s.