Why was the police dog licking his own asshole? To get the taste of nigger out of his mouth.
Why would you tell such an offensive joke about black people?
You're the racist. You are the one who doesn't get the joke.
Saying derogatory words like that is socially unacceptable, and could make you look really ignorant.  You should not tell such inflammatory jokes about other races, because not everyone is going to think it's funny and will get offended.