What did your last black slave die of?

I'm 18, and I am not racist. However, during my mother's generation, racist jokes were a normality. And the racism has obviously lingered. My mother's friend said to me last night 'Look, I'm black!' I stood there, very solemn and confused. At first I thought this was a joke about fake tan, but she didn't have any on. She noticed I was confused and said, 'what did your last slave die of? Don't you get it? Omg she doesn't get it' I get even more confused. She then explained to me that it was a widely known joke about the black slaves in the 1900's. For when a white person has to do any hard work for someone else, they say that sentence. First thing I said was 'that's incredibly racist and insensitive' I honestly hadn't heard a racist joke in years. Worst thing is. Is she is a psychologist. How can someone, who mindlessly overlooks racism as a past time for silly jokes, work in that industry?!it made me so mad! What do you guys think?have you ever heard of this before. Someone vent with me on how freaking ridiculous that is