Middle English derk, later dark, from Old English deorc "without light, lacking light or brightness (especially at night), obscure, gloomy;" figuratively "sad, cheerless; sinister, wicked," from Proto-Germanic *derkaz (source also of Old High German tarchanjan "to hide, conceal"), which is of uncertain etymology. For vowel change, see marsh.
Application to colors, "not radiating or reflecting much light," is from late 14c. Of complexion, "not fair," from early 14c. Figurative sense of "obscure, not easily understood" is from early 13c.; that of "sullen, sad" is from 1590s. Meaning "concealed, secret" is from late 14c. Dark Continent "Africa" (1828) combines several figurative senses (earliest references are in missionary publications). Theater slang for "closed" is from 1916.
Dark Ages "benighted time in history, period of ignorance" is attested by 1739; the specific focus on the centuries of the early Middle Ages in Europe, from the fall of Rome to the revival of secular literature, is from 1830s, from dark in a sense of "characterized by ignorance, backward in learning, void of intellectual light" (late 14c.).
Dark horse "competitor for honors or office about whom nothing certain is known, or whose identity is at first concealed," especially, in U.S., politics, "one who is unexpectedly brought forward as a candidate in a convention," 1842, is an image from horse racing, of horses whose performances or capabilities are not generally known, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
c. 1300, derken, "to make dark or darker, deprive of light;" early 14c. (intransitive), "to grow or become dark," from dark (adj.) + -en (1). The more usual verb in Middle English in both senses 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."
Meanings "grow less white or clear, turn a darker color" and "render less white or clear" are from late 14c. Figurative sense of "render gloomy, sadden" is from 1742. To darken(one's) door (usually with a negative) "enter one's house as a visitor," usually with an implication of unwelcomeness, is attested from 1729.
"in the dark," mid-15c., from dark (n.) + now-obsolete adverbial ending -ling (compare headlong). The verb darkle is a back-formation from 1810 (Moore, who rhymed it with sparkle), assuming the -ing as a present-participle adjective ending.
But having nothing to do with the participial -ing it does not mean growing dark &c.; from the mistaken notion that it is a participle spring both the misuse of the word itself and the spurious verb darkle. [Fowler]
By the same error, darkling (adj.), "dark, obscure, gloomy" is attested from 1763. The adverb was sometimes darklings, with adverbial genitive -s.
Old English deorcnysse "absence of light," from dark (adj.) + -ness. The 10c. Anglo-Saxon treatise on astronomy uses þeostrum for "darkness." Figurative use for "sinfulness, wickedness" is from early 14c. From late 14c. as "obscurity," also "secrecy, concealment," also "blindness," physical, mental, or spiritual.
Middle English dereling, from Old English deorling, dyrling "one who is much beloved, a favorite," double diminutive of deor "dear" (see dear (adj.)). The vowel shift from -e- to -a- (16c.) is usual for -er- followed by a consonant (see marsh).
As an adjective "very dear, particularly beloved," from 1590s; in affected use, "sweetly charming" (1805). "It is better to be An olde mans derlyng, than a yong mans werlyng" (1562).