https://www.cjr.org/analysis/capital-b-black-styleguide.php
From the article:
To capitalize Black, in her view, is to acknowledge that slavery “deliberately stripped” people forcibly shipped overseas “of all other ethnic/national ties.” She added, “African American is not wrong, and some prefer it, but if we are going to capitalize Asian and South Asian and Indigenous, for example, groups that include myriad ethnic identities united by shared race and geography and, to some degree, culture, then we also have to capitalize Black.”