Origin and history of magic

Old English blæc "absolutely dark, absorbing all light, of the color of soot or coal," reconstructed to be 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 usual Old English word for "black" was sweart (see swart).

The same root produced Middle English blake "pale," from Old English blac "bright, shining, glittering, pale;" the connecting notions being, perhaps, "fire" (bright) and "burned" (dark), or perhaps "absence of color." According to OED, in Middle English "it is often doubtful whether blac, blak, blake, means 'black, dark,' or 'pale, colourless, wan, livid' "; and the surname Blake can mean either "one of pale complexion" or "one of dark complexion." 

Black was used of dark-skinned people in Old English. Of coffee with nothing added, attested by 1796. The meaning "fierce, terrible, wicked" is from late 14c. The 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 drop (1823) was a liquid preparation of opium, used medicinally. Black-fly (c. 1600) was a name given to 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. Black flag, flown (especially by pirates) as a signal of no mercy, is from 1590s. Black dog "melancholy" is 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.

"one of the order of priests among the ancient Celts of Gaul, Britain, and Ireland," 1560s, from French druide (16c.), from Latin druis, fem. druias (plural druidae), from Gaulish Druides, from Celtic compound *dru-wid- "strong seer," from Old Celtic *derwos "true" (from PIE root *deru- "tree," especially oak) + *wid- "to know" (from PIE root *weid- "to see"). Hence, literally, perhaps, "they who know the oak" (perhaps in allusion to divination from mistletoe). Anglo-Saxon, too, used identical words to mean "tree" and "truth" (treow).

The tree, an enormous body between the fineness of its principles in the earth and the fineness of its aerial consequences. [Paul Valéry, untitled notebook, 1906, transl. Nathaniel Rudavsky-Brody]

The English form comes via Latin, not immediately from Celtic. Old English had dry "magician," presumably from Old Irish drui. The Old Irish form was drui (dative and accusative druid; plural druad), yielding Modern Irish and Gaelic draoi, genitive druadh "magician, sorcerer." Not to be confused with the United Ancient Order of Druids, a secret benefit society founded in London 1781.