Son: When I grow up I want to be a father. Dad: Fuck off son, you're black.
I don't understand what you're saying here. Are you making fun of someone's race?
It's not offensive and it's a joke. The joke is: do you really think black people can be good fathers?
It's not right to make jokes that imply that men are absent fathers who don't raise their kids because it's based on a racist stereotype. These kinds of jokes perpetuate ideas that are untrue and harmful. In addition, it doesn't make sense to generalize large groups of people.