Capitalizing “black” is a political statement, not a linguistic description. In English, races aren’t capitalized, while terms referring to distinct ethnic groups are (so you capitalize “German” but not “white”). As John McWhorter persuasively explains, rationale for capitalizing “Black” is that American descendants of slaves are a distinct group bound by shared history and culture. That’s fine insofar as “Black” is used to refer to what we might call “ADOS.” But in practice “Black” is used as a racial designator. At Harvard, for example, 40% of the “Black” students are african immigrants. Obama is “Black”—that describes his race, not his ethno-cultural group.
Capitalizing “black” is a political effort to center the experience of black people in American life. Consider the term “BIPOC,” which breaks out “black” and “indigenous” as first among equals even though they’re included in the “POC.” What is the intention of that?
These labels and classifications, in turn, have real world ramifications. My daughter’s school has a segregated “black girl magic” lunch every week. There’s no “half white half bangladeshi girl magic,” and few non-black, non-white students, so she was invited to attend the weekly lunch once a month. Even at age 12, my daughter is able to perceive there is a racial hierarchy designed to invert the historical one.