typical foreigner, getting all touchy about jokes!
Seriously tho, it's a tough call sometimes, but it seems that it's how it's told and who is telling it that makes the difference, whether it's right or not, nowadays most comedians are seen as having the knowledge and understanding about racism to know that it's not ok, and therefore can step over that line in an ironic or knowing way.
Whereas if you go back to the 70s and 80s, to the Bernard Manning and Jim Davidson era, it really was just making fun of 'those funny looking darkies', which isn't the same and is obviously a bad thing.
(Incidentally, I was going to use the term 'darkie' in my opening line, despite obviously not knowing anything about the above poster, it obviously being a joke about the whole topic, but bottled out as I felt it could be misconstrued even in a comedy thread, shame really.)