From black + -ly.
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.