The Chinese guys who were quite jealous and would not dare making parties themselves then had street fights with these Africans, and some of their racism also comes from such events in more recent times

Your post here demonstrates your extensive knowledge of China, it's people and society.

http://en.wikipedia.org/wiki/Nanjing_anti-African_protests

In Nanjing where I last taught there was this. There is also very strong anti japanese sentiment in China, especially so in Nanjing.

When I was teaching, anywhere in China, the boys would laugh and call me "se lang". At first I thought much like "lao wai" it was harmless and to be shrugged off, but then much like being called an old outsider, yes I believe "lao" can have negative connotations e.g. "lao shu" for example, I would actually find it quite annoying and rude. I did not like to be called a "Coloured wolf" simply because of my eye colour or whatever. Will take many, many more years for the Chinese education system to get rid of it's endemic racism.