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Webster 1913 Edition

adv.

In a black manner; darkly, in color; gloomily; threateningly; atrociously.

“Deeds so blackly grim and horrid.”

Feltham.

adv.

Darkly; atrociously.

blackly ‎(comparative more blackly, superlative most blackly)

In a black manner; darkly or gloomily.
- 1907, Harold Bindloss, chapter 26, in The Dust of Conflict:
  Maccario, it was evident, did not care to take the risk of blundering upon a picket, and a man led them by twisting paths until at last the hacienda rose blackly before them.