Three flavors. (1) Visual: gloomy, washed-out, pale (
惨淡的天空,
惨淡的月光). (2) Business/career: poor, struggling (
生意惨淡,
票房惨淡). (3) Effort: painstaking — only in the chengyu
惨淡经营 (cǎndàn jīngyíng), "to run [a business] through painstaking effort," originally from Du Fu's poem describing artistic creation.