When talking about colours both 乌 and 黑 seem to mean "black", what is the difference between them?
ex. there is 乌龙茶 but there is also 黑龙江, both 乌龙 and 黑龙 mean "black dragon".
乌 was a pictograph of a crow (烏鴉) . Since crow is black, the character 乌 was sometimes used as an adjective for 'dark' , 'black', or a verb for 'to blacken' in classical text.
黑 was a pictograph that depicted 'object being burned by fire' . It was used to represent the color 'black'. This meaning remain unchanged today.
In modern Chinese, 黑 is the primary word for 'black'(noun or adjective); 乌 is used for 'black' or 'dark' only in some set phrases , idioms and names
烏煙瘴氣- black smoke and miasma
烏雲 - dark cloud
烏衣 - classical version of 黑衣 (black clothes)