The colours which approach the dark side, and consequently, blue in particular, can be made to approximate to black; in fact, a very perfect Prussian blue, or an indigo acted on by vitriolic acid appears almost as a black.

—Johann Wolfgang von Goethe, Theory of Colours 

https://www.nytimes.com/2017/08/10/arts/design/glenn-ligon-rethinks-the-color-line-in-the-show-blue-black.html

Perhaps that’s because blue-black traces its roots back to a mythic point of origin in Africa, whereas “black,” along with “Negro” and “African-American,” might be considered just one more stopping point on the way to an as-yet-unknown destination.