descriptive · a deep, dark color that is black with a blue or purple tint, often used to describe skin discoloration due to cold or lack of oxygen

寒风中，他的脸色冻得有些乌青。

descriptive · an area of discolored skin on the body caused by a blow or impact (often categorized by the classifier 块 kuài)

摔跤以后，他的腿上起了一大块乌青。