"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.