What reason do you have to think it's a "slave food"? Peanut butter wasn't invented for some time after the end of the Civil War, and peanuts didn't become a major southern crop until after the Civil War either

Actually according to "Peanuts. The illustrious history of the Goober Pea" by Andrew Smith, peanut butter sandwiches were a food reserved for the rich. There was no industrial way of making peanut butter so it had to be hand ground which made it very expensive. They were commonly served in fancy New York City tea rooms. And interestingly the invention of sliced bread made it easy for children to make their own lunch which launched the popularity as a kids food. Finally something I know about in this thread!