In a lot of stories I've heard or seen, white people call black people by their first names as a way of belittling them. When I was a child the only adults that I was permitted to call by their first names were black Maids and Janitors, who were pretty much the only black people I knew. I realized in retrospect that this was racist, so I've always made it a point to call Black people I don't know well by Mr. or Ms. (or Dr. if they are academics.) The problem in California is that everybody calls everybody else by their first names, which probably makes me sound stuck up and formal. So if everybody else is using first names I usually go along with it. It's tricky to gauge when to do what.