The last time I was in China (Nanjing) I was friendly with some black guys from Cameroon, they were PhD students at a local university, but they also needed an income and were looking for teaching work. They just couldn't find any teaching, where as for us 'Europeans' there was, as always, a glut of schools looking for teachers. It is basically in my view embedded institutional racism rather than any government blacklist. Even the schools that looked the other way for people with tourist visas and no work permits wouldn't take them.