Ok, I'm in a bit of a rush but here goes...
Good question, though I feel compelled to point out that I never wrote that there was such a thing as a "black race". Nevertheles, you are, of course absolutely correct: there is no such thing as a "black race." There is, I think, such a thing as a broad "Black" culture in which, to be clear, I DO NOT include the various cultures of Africa. "Black culture" is produced and reproduced in the Americas and is multiply manifest: music (blues, gospel, jazz, hip-hop, rap, reggae, samba, etc.) literature (Baldwin, Morrison, Hughes, etc.) language (Black English, Gullah, creole, rap, jazz poetry) art (Palmer Hayden, Archibald Motley, William Scott, Clementine Hunter, etc.)...That's why I capitalize "Black."
As you pointed out, the term "Black" is obviously problematic. Sometimes folks use it to mean "culture," sometimes "race," sometimes "nationality," sometimes "ethnicity," sometimes "political identity," or any mixture thereof. Clearly, in some ways, the use of the term "Black" generates more confusion than clarity (not to mention lots of useless Afro-centric idealogical tendencies).
Furthurmore, it may not be appropriate to label all of the stuff listed in the first paragraph as products of Black culture. It is true that the primary (not by a long shot the only) engine of national cultural production in those countries that have a history of racial slavery are those people descended from slaves. But then, what beside the various degrees of political repression associated with their skin colors do rastafarians, New York hip-hopsters, and fundamentalist Southern Baptist preachers have in common? Does calling that "Black culture" hide some critical, subtle differences between these groups? Probobly. There are lots of other questions to ask. The pursuit to come up with more precision language that facilitates a precision material understanding of the relationship between cultural production and political econonomy needs to continue.
At bottom, though, I capitalize "Black" because it has a healthy moral, ethical, cultural and idealogical substance that "white" does not. In fact, contrary to a lot of these silly "white studies" types, there is no such thing as a "white" culture or, for that matter, a positive "white" identity. When people start talking about "feeling great about being white" I get (among other things) nervous. Or, to put it another way, "white" power is a really different thing than "Black" power.
So in short, I categoricly and publicly reject the notion of a "black race," gleefully embrace my Black political identity, and accept the notion of a Black (Afro-American?) culture tho that concept needs some work.
Ok, there's my five minute ramble. I'm sure I can do better later. Does that help or confuse?
Niles