5 times or more, none in recent history.
When I lived in Chicago, I had constant problems with them as there were a lot wearing all red or blue clothing as they were in gangs. I had more issues with the Mexicans than the blacks.
In Chicago, it was more psychological, I didn't have too much of a problem with the blacks, but southern blacks are some of the most racist [censored] you will meet.
When I moved to Florida, I was a junior in highschool I had a problem with one black kid that I recall when I first moved here, was much taller than me (about 6ft5). I was on one of the computers doing work in the classroom and the black kid just said "MOVE!" and I responded saying "i'm using it". He ended up grabbing the chair and tossing me out of it and I was thinking "what the [censored] is your problem?" teacher didn't say anything and no one noticed a thing. The next few days, kept harrasing me, when we were leaving the classroom, would toss me to the flooror if I was sitting at the table (we didn't have desks in the classroom), he would constantly push my chair into the table while I was sitting or throw pencils at me. One day I had enough and he and his Mexican lackey and white traitor friends were throwing pencils at the back of my head. I got out of my chair, picked up the chair and threw it at him, everyone was like "oooooh!!!", teacher comes in the classroom and the black [censored] is jumping over the table grabbing me about to punch me. Both of us are sent to deans office, the black kid is suspended as he had a history of violence, i'm surprised how long I tolerated that crap.