There is a book being written about Jacob and his brother John. According to the authors' research, John left the Amish Church and moved south. He became a slave owner. His son, Reuben, fathered a child with a slave girl. That boy was named Charley Mast, and is the great-great-grandfather of one of the authors. The other author comes from the Jacob Mast family line.
The book is called "Kinship Concealed: Amish Mennonites/African-American Connections."
Here's a link to an article about the book and authors.