I use capital "B" and lowercase "w" for that reason too. (I'm white.) My understanding is that many Black people experience their Blackness as an ethnicity, as white people stole and erased the heritage of many people who are today called Black, and Blackness develops its own cultures and solidarity too. Of course it's not monolithic and doesn't have to be. Nor is it mine to question or take away from anyone.

My ethnicity is Eastern European Jewish. The letters are capitalized because it's a trait I can't change, and it doesn't hurt anyone, and I'm allowed to appreciate it and be proud of it. By contrast, I'm white because I have white privilege, and I lowercase "white" as a reminder that whiteness is a power structure and a social dynamic that needs to be dismantled. No need to reify whiteness. White people need to acknowledge our whiteness so we can work on stopping the harm we cause through it, essentially to try to become less white. It's not an umbrella ethnicity. I don't feel kinship or solidarity with other people of European descent just because we're white—and if I ever do, that's the sort of prejudice I'd need to interrupt. Therefore, lowercase "w."

But I am interested in hearing others' opinions, which is why I'm reading. It's symbolism, so it's a matter of collective interpretation.