https://www.washingtonpost.com/pr/2020/07/29/washington-post...
>AP’s style is now to capitalize Black in a racial, ethnic or cultural sense, conveying an essential and shared sense of history, identity and community among people who identify as Black, including those in the African diaspora and within Africa.
Africa is larger in both land area and population than Europe and the United States combined. Black Africa is a population of greater linguistic [0], religious [1], cultural [2], and genetic [3] diversity than the white populations of Europe or the US. To the extent that a unified black identity exists, a unified white identity most exist also. Inventing one demonym to cover all black peoples of Africa and America, but not one that covers white Americans and/or Europeans, is not a decision based on history or evidence. It is purely an ideological decision, made through a uniquely American lens.
[0] https://www.csmonitor.com/Science/Science-Notebook/2015/0421...
"With more than 2,000 distinct languages, Africa has a third of the world's languages with less than a seventh of the world's population. By comparison, Europe, which has about an eighth of the world's population, has only about 300 languages."
[1] https://www.pewforum.org/2014/04/04/global-religious-diversi...
"12 countries have a very high degree of religious diversity. Six of the 12 are in the Asia-Pacific region; five are in sub-Saharan Africa; and one is in Latin America and the Caribbean. No countries in Europe, North America or the Middle East-North Africa region have a very high degree of religious diversity as measured in this study."
[2] https://www.pewresearch.org/fact-tank/2013/07/18/the-most-an...
"The usual suspects lead the list of culturally diverse countries: Chad, Cameroon, Nigeria, Togo and the Democratic Republic of the Congo. These and other African countries typically rank high on any diversity index because of their multitude of tribal groups and languages. The only western country to break into the top 20 most diverse is Canada."
[3] https://www.bbc.com/news/science-environment-30318291
"Until now, most studies examining genetic risk factors for disease have focused on Europe. Little has been known about Africa, the most genetically diverse region in the world."
Without debating either of those points, how are they relevant to grammatical rules. The words are either proper nouns or they are not.