乌云 is often used figuratively: 心头的乌云 (the dark cloud in one's heart), 乌云笼罩 (dark clouds loom — a metaphor for gloomy situations). 乌云密布 (wūyún mìbù) means dark clouds are thick and dense.
culture
乌 originally means 'crow' (乌鸦, wūyā), which is black. So 乌 came to mean 'black/dark.' This connection is why 乌云 literally means 'crow-colored clouds.'
