黑 and 乌 talking about colours

Question

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".

Answer 1

http://www.zdic.net/z/1d/sw/70CF.htm

孝鳥也。象形

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.

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火所熏之色也

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

Examples:

烏煙瘴氣- black smoke and miasma

烏雲 - dark cloud

烏衣 - classical version of 黑衣 (black clothes)