The Collaborative International Dictionary of English v.0.48:

Darkling \Dark"ling\ (d[aum]rk"l[i^]ng), adv. [Dark + the
   adverbial suffix -ling.]
   In the dark. [Poetic]
   [1913 Webster]

         So, out went the candle, and we were left darkling.
                                                  --Shak.
   [1913 Webster]

         As the wakeful bird
         Sings darkling.                          --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Darkling \Dark"ling\, p. pr. & a.
   1. Becoming dark or gloomy; frowing.
      [1913 Webster]

            His honest brows darkling as he looked towards me.
                                                  --Thackeray.
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   2. Dark; gloomy. "The darkling precipice." --Moore.
      [1913 Webster]

WordNet (r) 3.0 (2006):

darkling
    adj 1: uncannily or threateningly dark or obscure; "a darkling
           glance"; "secret operatives and darkling
           conspiracies"-Archibald MacLeish
    2: (poetic) occurring in the dark or night; "a darkling journey"