darkling (adv.)youdaoicibaDictYouDict[darkling 词源字典]
"in the dark," mid-15c., from dark (n.) + now-obsolete adverbial ending -ling (compare headlong).
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]
[darkling etymology, darkling origin, 英语词源]