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.
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