It's like all the anime characters are blonde or red haired with white skin.
Most Helpful Guy
... Most anime characters are Asian ethnically. The culture just views pale skin as attractive so it shows up more often than not. Hair dye is also pretty readily available, and there is a tendency to use bright colours for hair as a stylistic choice rather than as any representation of realistic colour.
As for black characters... most people who have spent time in Japan will admit the culture is pretty xenophobic. They sometimes idolise europeans or americans in an abstract sense, hence there being a token non-japanese in virtually every show, but less on the individual... I assume by 'africans' you mean black people, rather than people from the continent (who simply dont appear often because its a long way away, the same as south americans rarely appear on British tv). Black people just arnt something many Asians are exposed to... and some would argue require quite different colouring/animation techniques to look right. Still there are plenty of blacks as minor characters in anime, usually as American military personel (which sometimes has its own obvious, negative, conotations)... want a list?0
Most Helpful Girl
A lot of the characters aren't actually white, their skin is pale but they're not white. Blonde and red hair is used to indicate interesting, important or quirky characters because that's unusual among Japanese people and it highlights which characters you're supposed to pay attention to.
There are some black anime characters such as Ed from Cowboy Bebop, Garter from Panty and Stocking and a few of the characters from One Piece. I also think Integra from Hellsing was black even though she doesn't appear to be in the pictures alone but with others it's more noticeable, either way, she's described as having 'creamy brown skin'.
I assume that the reason for so few black people is that either the animators find it difficult to show darker skin tones (though if we look at Garter, he is definitely dark skinned so they can't always use that excuse), or that as of July 2015 '98.5% of the population is ethnic Japanese, 0.5% Korean, 0.4% Chinese, others made up 0.6%.'
When you think of how many races have to make up that remaining 0.6% and how that means not even 0.6% of the population of Japan is black, I assume the animators simply decide that there aren't enough people to appeal to and therefore don't include very much diversity.0