There are some really nice books in the library in the children's nonfiction section that discuss our human differences and similarities. Ask the librarian for books that are appropriate for kindergarteners and be sure to flip through them before reading aloud. See if that helps her understand the beauty of humanity. Will see if I can find my list of books for my 5-year-old and send offline if you need that.
As for her African American heritage, you might want to read African Beginnings (for yourself), also in the library, which gives information about various kingdoms prior to and through slavery. I would take this as an opportunity to show her Africa and Asia on the globe and how far it is from America. Teach her that people from all over the world came to America and brought with it their good and bad ways of doing things. Slavery was a worldwide system. And, once slavery ended in America, attitudes about each didn't change and laws were created that were not fair. Tell her that as she gets older, she will learn more about all of these things in history (which, sadly, you might have to teach at home). Anyway, teach her that there are many people who were many heroes because they fought against that unfairness, some Black like Rosa Parks, and some White like John Brown, and you will be more than happy to introduce her to biographies.
I think this is a wonderful opportunity to teach her about choices people make and how they hurt so many other people and hinder us from really loving one another. Teach her that from these negative incidents in history, many people have risen above their situation. Introduce (or re-introduce) to her those who have fought those injustices so that she will be proud of her heritage. It is not a shameful past that she has inherited, but one that she can embrace with pride, dignity and courage.