Initially being a gaijin in Tokyo is a little difficult irrespective of whether you're black/white/brown because of language problem, cultural differences and different food habits. But in general, people are helpful- there's no discrimination. Sometimes you may feel that people are ignoring you or are trying to avoid speaking to you- that's more because of the language problem though. So if you can speak Japanese, it will be quite easy. And yeah- Japanese girls love gaijins