It was a reminder of what a recent thing slavery is. The guy in Mississippi was almost the last exponent of a style called drum and flute- there never were very many of them because the drum was banned (on pain of death) from the old plantations. I say "almost" because his daughter or granddaughter, still only a kid when the film was made, is carrying on the family tradition. Listening to him- and her- you were whisked right back into the 18th century- a matter of (what?) two, three, four generations ago.
The crime of slavery is almost too huge to comprehend. But those who perpetrated it had almost no idea- no idea at all- that they were in the wrong.
How stupid human beings are. How Willfully stupid. The slave-owners had no idea because they wouldn't allow the idea to form. But they weren't all-round stupid. Jefferson was a slave-owner- and one of the most fiercely intelligent (and in certain spheres) noble men who have ever lived.
It wasn't that the idea (that slavery is wrong) was unavailable to them. Other people were perfectly well able to formulate it. Dr Johnson, for example. Johnson the monarchist was against slavery and Jefferson the apostle of liberty was in favour.
The tribal chief in Mali said, "there are no black Americans. There are only black people who happen to live in America." For him the exodus of the slaves to north America is something fresh and new- recent history- a living affront. He and our American guy in dreds sat under a tree and jammed together. They sang alternate verses. They were singing in their two different languages to the same tune.