verb tr., intr.: To make or become dark, indistinct, or gloomy.
Back-formation from darkling (in the dark), from Middle English derkeling. Earliest documented use: 1819.
“The silhouettes of builders and road-construction equipment darkled against the sky.”
Dovletmurad Orazkuliev; New Roads in Country; Neitralnii Turkmenistan; Jul 6, 2010.