1. 
[syn: darkly, in darkness]
2.  in a dark glowering menacing manner; 
- Example: "he stared darkly at her"
The Collaborative International Dictionary of English v.0.48:
Darkly \Dark"ly\, adv.
   1. With imperfect light, clearness, or knowledge; obscurely;
      dimly; blindly; uncertainly.
      [1913 Webster]
            What fame to future times conveys but darkly down.
                                                  --Dryden.
      [1913 Webster]
            so softly dark and darkly pure.       --Byron.
      [1913 Webster]
   2. With a dark, gloomy, cruel, or menacing look.
      [1913 Webster]
            Looking darkly at the clerguman.      --Hawthorne.
      [1913 Webster]
WordNet (r) 3.0 (2006):
darkly
    adv 1: without light; "the river was sliding darkly under the
           mist" [syn: darkly, in darkness]
    2: in a dark glowering menacing manner; "he stared darkly at
       her"