early 13c., "white, pale, colorless," from Old French blanc "white, shining," from Frankish *blank "white, gleaming," or some other Germanic source (compare Old Norse blakkr, Old English blanca "white horse;" Old High German blanc, blanch; German blank "shining, bright"), from Proto-Germanic *blangkaz "to shine, dazzle," extended form of PIE root *bhel- (1) "to shine, flash, burn," also "shining white."
Meaning "having empty spaces" evolved c. 1400. Sense of "void of expression" (a blank look) is from 1550s. Spanish blanco, Italian bianco are said to be from Germanic. Related: Blankly, blankness.