To make everything clear what the reasons are why Zwarte Piet was made up and why he's black here's a little video about it. Earlier than this people were making their faces black or white for ages already.
There's also something positive about the why of the earrings, the many Zwarte Pieten, the curly hair, the surprises, etc. You can see it on the dutch website too: www.zwartepietisgoed.nl