黑 and 乌 talking about colours
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)