First, let's start with a bit of an English grammar lesson. My name, Alex, has the A capitalised. This isn't because I'm uniquely important — your name (if written with a Latin script) is also written with a capital. This is because it is a label. It's a descriptor that has been attributed to me. One that I happen to like so I haven't, as-of-yet, changed it. The grammar rule therefore is: proper nouns should have their first letter capitalised.
Now, Blackness and being Black is not a neutral biological fact. It is not the same as the colour black. Being Black comes from our culture, our history, our communities, and our unique connection to the continent of Africa compared to other residents and diasporas of their respective continents of origin. As you can tell from the picture of me on the website, you know that when I refer to my spotless (literally), wrinkle-free, Black skin, you know I'm not referring to its colour as I'm very much on the lighter-side. Thus the capitalisation of the B can also help signify to readers that we're talking about the pseudo-scientific racial classification based on our phenotypical traits and cultures.
For this reason, I'll be capitalising the B in Black when referring to race. I hope you'll join me in popularising this.