When I was at school, there were two types of blackboard. Some were a large wide belt over rollers, effectively doubling the surface area. They weren't made of slate. They were black. Some were a fixed board. They weren't made of slate. They were black. Many materials that aren't slate are black. Black is a good background colour when the writing is white and needs to be seen clearly by a group of people.
The root of the problems isn't the colour black. It's the belief that every reference to black or white is about "race" and it's the belief that black and white make any sense at all as descriptions of people and it's the belief in the whole idea of "race".
Geez! I hated the roller ones. Saying that I detested the flat ones as equally as everytime I hear the chalk scrape off them it made my skin crawl. Plus hate the smell off the chalk and dusters.
There was always some prat that dragged their nails down them. That finished me off.