It's pretty sad to see how a lot of Blacks are portrayed in anime/manga with their gross, overplayed stereotypes. It seems that usually only whites are portrayed well in anime/manga, despite stereotypes.
And another thing - whenever there is a mixed blood character, they are NEVER half black...And very rarely half anything but white.
I think it was Go Nagai who created the first mixed race character, but I have no clue who it is or what anime s/he was in. I think it was just a minor character, though.
Other than that, I think the portrayal has only become slightly better... in terms of looks that is. Decades ago they were drawn horribly.
Can't say much about the stereotypes, I've not watched that much anime with black people in it.
In quite a few mangas to this day (especially if the setting is foreign), blacks are still drawn badly, with overly exaggerated features, bad behaviour etc.