Being well defined is also not a usual reason to capitalize something, and saying it should be capitalized because it's a proper noun is just circular logic. Why do you think it's important that there's an accepted grammatical rule that is going to explain capitalizing Black but not white? If you want to capitalize it to emphasize the shared history of Black people, then just do it. You don't need the academy to approve. Do you think bell hooks was like paging through Strunk & White until she found the loophole that would allow her to decapitalize her pen name?