Sidling in the Darkling Night

   Darkling began its life as an adverb meaning 'in the dark'.  ('Out went the candle and we were left darkling' said the Fool in King Lear).

Keats, in 'Ode to a Nightingale', wrote 'Darkling I listen...'  meaning he was standing in the dark, listening.

But even before Keats used it in its proper form the word had come to be regarded as the participle of a verb 'to darkle'.  And ever since things have darkled for the poets so we find that night darkled, moonles landscapes and even minds have done likewise. 

This creation of a verb from an adverb is called back-formation and a much more common example is the verb to sidle, meaning to walk in a furtive or timid manner, especially obliquely.  Originally there was no such verb but it was back-formed from the adverb side-long or side-ling.