It might stem from Uber's roots, since all of the Uber Black cars are by definition black. It is pretty annoying when using their billions of other offerings -- not everyone can spot a particular Honda Civic from two blocks away.

I took a white town car on UberBLACK back in the day. It was pretty cool, it had a disco ball and pink lights inside.

Interesting, I've used it a bunch over the years and never encountered a non-black car. Which is actually kind of silly when you think about it -- a silver Mercedes is just as classy as a black one, if that's what you're concerned with (I'm actually more interested in getting a livery driver who knows what they're doing).