Darkling comes from Middle English derkelyng, and the verb darkle is a back formation thereof. As a noun it means either darkness or a (fantasy) creature that lives in the dark. It can also appear as an adjective meaning dark or darkening, or something that is obscure, unseen, or happening in the cover of darkness. As an adverb it means in the dark or obscurity.
There is a Darkling Beetle, and a poem by Thomas Hardy called The Darkling Thrush, though the more usual use of the word is to be found in Science Fiction, e.g. in Star Trek Voyager, Marvel Comics, and a wide range of fantasy characters on the dark side of the fence.
According to the Urban Dictionary, you are a darkling if you are more sarcastic than charming, or if you are a geek, but a cool one. Another application might be a portmanteau word from dark and darling.