Entries linking to blacken
Old English blæc "absolutely dark, absorbing all light, the color of soot or coal," from Proto-Germanic *blakaz "burned" (source also of Old Norse blakkr "dark," Old High German blah "black," Swedish bläck "ink," Dutch blaken "to burn"), from PIE *bhleg- "to burn, gleam, shine, flash" (source also of Greek phlegein "to burn, scorch," Latin flagrare "to blaze, glow, burn"), from root *bhel- (1) "to shine, flash, burn."
The same root produced Old English blac "bright, shining, glittering, pale;" the connecting notions being, perhaps, "fire" (bright) and "burned" (dark), or perhaps "absence of color." "There is nothing more variable than the signification of words designating colour" [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859].
The usual Old English word for "black" was sweart (see swart). According to OED: "In ME. it is often doubtful whether blac, blak, blake, means 'black, dark,' or 'pale, colourless, wan, livid.' " Used of dark-skinned people in Old English.
Of coffee with nothing added, attested by 1796. Black drop (1823) was a liquid preparation of opium, used medicinally. Black-fly (c. 1600) was used of various insects, especially an annoying pest of the northern American woods. Black Prince as a nickname of the eldest son of Edward III is attested by 1560s; the exact signification is uncertain.
Meaning "fierce, terrible, wicked" is from late 14c. Figurative senses often come from the notion of "without light," moral or spiritual. Latin niger had many of the same figurative senses ("gloomy; unlucky; bad, wicked, malicious"). The metaphoric use of the Greek word, melas, however, tended to reflect the notion of "shrouded in darkness, overcast." In English it has been the color of sin and sorrow at least since c. 1300; the sense of "with dark purposes, malignant" emerged 1580s (in black art "necromancy;" it is also the sense in black magic). Black flag, flown (especially by pirates) as a signal of "no mercy," is from 1590s. Black dog "melancholy" attested from 1826.
Black belt is from 1870 in reference to district extending across the U.S. South with heaviest African population (also sometimes in reference to the fertility of the soil); it is attested from 1913 in the judo sense, worn by one who has attained a certain high degree of proficiency. Black power is from 1966, associated with Stokely Carmichael. Black English "English as spoken by African-Americans," is by 1969. The Black Panther (1965) movement was an outgrowth of Student Nonviolent Co-ordinating Committee. Black studies is attested from 1968.
word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.