bleach (v.)
Old English blæcan "bleach, whiten," from Proto-Germanic *blaikjan "to make white" (cognates: Old Saxon blek, Old Norse bleikr, Dutch bleek, Old High German bleih, German bleich "pale;" Old Norse bleikja, Dutch bleken, German bleichen "to bleach"), from PIE root *bhel- (1) "to shine, flash, burn" (cognates: Sanskrit bhrajate "shines;" Greek phlegein "to burn;" Latin flamma "flame," fulmen "lightning," fulgere "to shine, flash," flagrare "to burn;" Old Church Slavonic belu "white;" Lithuanian balnas "pale").

The same root probably produced black; perhaps because both black and white are colorless, or because both are associated with burning. Compare Old English scimian, related to the source of shine (n.), meaning both "to shine" and "to dim, grow dusky, grow dark." Related: Bleached; bleaching.
bleach (n.)
"act of bleaching," 1887; "a bleaching agent," 1898, probably directly from bleach (v.). The Old English noun blæce meant "leprosy;" Late Old English also had blæco "paleness," and Middle English had blech "whitening or bleaching agent."