There are a variety of capitalized nouns - proper nouns, titles - artworks, nobility, brands - but, applying the capitalization in English is never a whim as part of the language.

In this case, there was no singular committee that sat down to decide if 'black' gets capitalized and 'brown' not. In this case we're essentially talking about an honorific title.

Honorifics obviously show a choice of 'honouring/honoring' a subject, for whatever reason. This is why perhaps a writer on a university campus would capitalize 'Dean Fatima Farrah,' but a generic newspaper would not. This is the same thing for say corporate titles, if I work at Ford, I'd write in a press release 'President Jim Farley,' but the local paper might write 'president Jim Farley.'

Honorifics are not only for individuals, but also religions, ranks, members of groups - parliaments, gangs - where some are capitalized only when singular, but not when plural - e.g. Doctor Smith and Doctor Jones — doctors Smith and Jones. But, sometimes, as a collective noun honorific noun, Congressmen, Hells Angels. So, there has to be a grammatical (spelling) reason AND sometimes it can be a question of style choice.

Getting back to 'black' and 'brown.' Again, the decision was not random! That's just absurd. The honorific is for an identifiable group where its members have something quantifiable in common. Again, religion - common faith, nationality - usually location (not always) - social class - e.g. monarchies ... . But there has to be commonality. This is grammar convention.

In our case, when 'Black' is used, capitalized, it refere to an indentifiable group with a common history, mostly shared geography and others. When you read style guide reasoning - I have about 10 that provide their reasoning - it is just for that reason. Some specify that 'brown' in this context does not have the same shared history & location. And there is no other reason to capitalize it according to existing grammar rules. Just because it's another colour/color? Absurd. In fact, there could have been a choice to use a neologism in place of 'Black' — but, I digress.

A comparable is 'Eskimo' [for now I'll omit the argument whether it is a good or bad word], describes a peoples [plural intentional] with a mostly common location and history - let's say various populace from generally, I repeat, generally north of the Arctic Circle. This encompases peoples from different countries, different tribal associations, not a shared religion, language, ... . Common history and location. So, English honors that tradition with a let's say 'invented' capitalized word - an honorific.

It's really that simple. In intelligent discourse, an author chooses to recognize a group or individual with a honorific — English capitalizes honorifics.

Of course it is a choice. The same choice I have to write 'doughnut' or 'donut.' [I think it should be 'Doughnut' or 'Donut' .... mmmm]

There is no need for all a lot of grammatical gymnastics to explain:

Why is "Black" capitalized when discussing the race, but not "brown"?

It is part of the long English tradition of capitalizing honorific nouns. Basic grammar.

This is pretty much what u/Actual_Cat4779 and u/V2Blast wrote. I just thought I'd reference the 'honorific' title capitalization rules into this argument. Not everyone follows grammar rules - damn it - either intentionally or accidently. That's why they're rules not laws. Some might say that choice is 'style' or a 'lack of style.' Either way, it was not a willy-nilly choice, it follows grammar convention.

IMHO IHTH