once you go black , you 'll sit in the back
What do you mean to imply with this phrase?
African Americans are lazy, they don't want to work and sit in the back of the bus and take up space
Jokes like this are reminiscent of a time when black people were discriminated against legally, and this is not something that we should be celebrating.  If someone on the bus appears to be lazy, they are likely exhausted from working multiple jobs without the kinds of privileges that you and I take for granted, such as having a car and having had access to a good education.  This is not a moral failing on their part, it is a sign that they are doing the best they can.