bleach (v.)youdaoicibaDictYouDict[bleach 词源字典]
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 etymology, bleach origin, 英语词源]
darken (v.)youdaoicibaDictYouDict
c. 1300, "to make dark;" late 14c., "to become dark," from dark (adj.) + -en (1). The more usual verb in Middle English was simply dark, as it is in Chaucer and Shakespeare, and darken did not predominate until 17c. The Anglo-Saxons also had a verb sweorcan meaning "to grow dark." To darken someone's door (usually with a negative) is attested from 1729.