I will try to get us back on topic.
I can't say if it is the same in China and Korea as in Tokyo or even if it's the same in other parts of Japan but in Tokyo, a fair percentage of black people that the public would encounter on a day to day basis work in the entertainment districts that are popular with foreigners. Their jobs are to approach tourists and expats walking down the street and try to convince them to come into their bars/clubs/gentlemen's clubs. After they try to sell you on a specific club, they switch to trying to either sell you drugs or girls. After dark on the main drag in Roppongi there are at least a hundred of these guys out on the sidewalk. Most of them are agressively friendly and don't give up easily. All of the guys I've talked to are directly from Africa and I have been told by locals that they are brought in by the mafia to sell drugs and pimp girls to the tourists. Someone with connections is either getting them visas or getting around the immigration system so I would not be surprised that it would require the Yakuza's involvement.
If in China, africans (I don't know anything about mongolians) are also brought in to sell drugs and prostitutes to foreigners then the crackdown for the Olympics would at least have some logic behind it. I'm not defending the practice, btw. I think that this Olympics is going to be one PR disaster after another. How sad that Toronto lost out to this bid and to the Coca-Cola Games in Atlanta.
I will add that I have no idea what percentage of the black people in Tokyo at any time are either pimps/drug dealers or tourists or people with respectable jobs (There are people from all over the world working in my office.) but I think that these guys are visible enough to affect the public image of all black people to your average Tokyo-ite.
As for racism in Japan, I have very little doubt that it exists. In Tokyo, there are definitely clubs that are only for Japanese or for 'respectable' white people with Japanese friends but I haven't seen any "No Whites" or "No Blacks" signs.
I have heard of "No Blacks" signs in the south of Japan. I have been told that anti-foreigner racism is more common in the south and that there is probably a link between that and the large number of US military in the south. Again, this is all second hand information. The most racist thing that I've heard here was that the black guys working at the clubs weren't just brought in by the Yakuza to sell drugs but to spread AIDS among the Japanese people - but this bit of urban mythology was told to me by a Chinese-American from Boston.
I'm not sure how people who believe this shit think the Yakuza would benefit from this.
1. Import Africans
2. Spread Aids
3. ????
4. Profit!
Anyways, I thought that I would put in my 2 cents. Oh, and Japanese people eat raw horse.