One of the Bulgarian players was black. He had South African ancestry. I am guessing probably his parents left during apartheid and thought that the Communist block would be attractive.
He didn't get booed, but I bet those idiot rightwingers don't like having a black Bulgarian players, rather like our rightwingers didn't like black English players not that long ago.
Mind you, I bet those knobs don't like it if someone of Romanian or Hungarian or Albanian or Romanian heritage plays for them. PLaces like Bulgaria were always very multicultural, until the First World War, and then the "nation-state" fetishists got their way.