Origins
The English word 'black' has one of the most paradoxical etymologies among common words. It descends from Old English 'blæc,' meaning dark or ink-coloured, from Proto-Germanic *blakaz, meaning 'burned' or 'charred.' The ultimate PIE root is *bʰleg-, which meant 'to burn, to shine, to flash.' This root captures a genuine duality: fire both shines brightly and leaves behind charred, blackened remains. The English word 'black' descends from the charring sense; the brightness sense survives in related words from other branches.
The PIE root *bʰleg- produced Latin 'flagrāre' (to blaze, the source of English 'flagrant,' originally meaning 'blazing'), Greek 'phlégein' (to burn, as in 'phlegm,' originally 'inflammation'), and Latin 'fulgēre' (to flash, the source of 'refulgent'). These cognates all preserve the 'bright, burning' sense of the root, while the Germanic branch uniquely developed the 'burned, darkened' sense.
Old English complicated matters by preserving both senses in near-homophonous words. 'Blæc' (with a short vowel) meant 'black, dark' — from the charring sense. 'Blāc' (with a long vowel) meant 'bright, shining, pale' — from the illumination sense. The two words were frequently confused by scribes, and context was often the only way to determine which meaning was intended. This confusion contributed to the eventual disappearance of 'blāc' in the bright/pale sense, leaving only 'black' in its dark meaning. The 'pale' sense survives vestigially in the verb 'bleach,' which descends from Old English 'blǣcan' (to whiten), from the same root family.
Proto-Indo-European Roots
The relationship between 'black' and 'bleach' is thus genuinely etymological: both trace to the PIE fire-root *bʰleg-, with 'black' representing what fire destroys and 'bleach' representing the lightening action associated with the root's brightness sense. 'Blank' (from Old French 'blanc,' white, ultimately from Proto-Germanic *blankaz, shining) is likely from the same extended family.
In the Germanic languages, the words for 'black' diverge notably. German uses 'schwarz' (from Proto-Germanic *swartaz, related to English 'swarthy'), not a cognate of English 'black.' Dutch uses 'zwart' (same origin as German 'schwarz'). The Scandinavian languages use 'svart' (Swedish, Norwegian) and 'svartur' (Icelandic). English is unusual among Germanic languages in having adopted the 'blæc/burned' word rather than the '*swart-' word as its primary term for black. Old English did have 'sweart' (dark, black), cognate with these other Germanic forms, but 'blæc' prevailed — 'swart' and 'swarthy' survive in English only as literary or archaic terms.
The phonological development from Old English 'blæc' to Modern English 'black' is regular. The short vowel /æ/ remained essentially unchanged through Middle English (spelled 'blak' or 'blac'). The modern spelling with 'ck' was standardized in early Modern English.
Figurative Development
Semantically, 'black' has accumulated an enormous range of metaphorical meanings in English, many of them negative: blackmail, blacklist, black market, black sheep, black mark, black mood. These associations are culturally constructed rather than etymological — the word's original meaning was simply 'charred' or 'burned,' with no moral connotation. The 'blacksmith' is so called not because of moral darkness but because the smith works with 'black' metals (iron and steel), as opposed to a 'whitesmith' who works with 'white' metals (tin, pewter, silver).
Black Friday, in its modern commercial sense, dates only to the 1960s in Philadelphia, where police used the term to describe the chaotic day after Thanksgiving. The popular etymology connecting it to businesses moving 'into the black' (profitability) is a later back-formation. The term 'in the black' itself comes from bookkeeping, where profits were recorded in black ink and losses in red.