Sentence:  The queen's letter coming up to the duchess's own ideas of her own deserts, she condescended to speak on the subject which had caused such portentous blackness to <WSD>lour</WSD> on her countenance, on her first meeting her royal mistress. 
Meaning of the word marked with <WSD> tags:  To be dark, gloomy, and threatening, as clouds; of the sky: to be covered with dark and threatening clouds; to show threatening signs of approach, as a tempest.