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.
"greenish-blue color," 1889, short for cyan blue (1879), from Greek kyanos "dark blue, dark blue enamel, lapis lazuli," probably a non-Indo-European word, but perhaps akin to, or from, Hittite *kuwanna(n)- "copper blue."