Actually, American history was one of my best subjects in high school and one on which I did (unexpectedly) superior SATs. I don't need any education from you on the topic.
Not to defend MANDINGO, but there are certainly first-person slave narratives that relate tales of sexual abuse aplenty. See e.g., Harriet Jacobs' Incidents in the Life of a Slave Girl, reprinted in the Library of America volume "Slave Narratives" and described briefly in Harriet Ann Jacobs - Wikipedia, the free encyclopedia.
It's been far too long since I've seen Mandingo to recall any particular kinks to do further checking, though. I feel like I need to wash now.