mid-14c., bluschen, blischen, 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 (e.g. Dutch blozen "to blush") and Scandinavian (e.g. Danish blusse "to blaze; to blush"); ultimately from PIE *bhel- (1) "to shine, flash, burn" (see bleach (v.)).
For vowel evolution, see bury. Earliest recorded senses were "to shine brightly; to look, stare." Sense of "turn red in the face" (with shame, modesty, etc.) is from c.1400. Related: Blushed; blushing.