How does the Japanese society view African-Americans?

I don't know about japanese, but I've seen my black friend discriminated against many times by old Korean people. I would say that the younger generation is more connected to the world and are open to different races, but get out into the rural areas of Japan and you might see a different story.

if you go to the city its fine. there are tons of foreigners people are use to it already. especially around yokohama you will see lots of half jp/blk people

In my experiences, Japanese tend to be more judgmental against anyone who aren't Japanese or white. That being said, I know quite a few people (black) who are married to Japanese. So with all that said, don't be afraid, the people who judge and hate based on skin color are a minority here in Japan. You will have more issues than I have I believe.