Here in the South, slaves frequently took the last name of their owner. The slaves basically had no 'last name' so when they were freed the slave just took the name of their last owner. Therefore, you would have have large populations in one area with the same last name, both white and black.
Yes, I know. I'm in NC. One of xoxox's posts seemed to indicate that many of the black Mangums had a European gene present in the DNA, which would indicate mixed bloodlines.