Slavery was a very different affair in the days of our founders and it had changed considerably from the 1650s or so when it rose out of indentured servitude. Even this wiki account of the life of Washington’s personal slave gives the impression that Billy Lee was far more than a slave. He had a better life than poor white farmers of the day.
http://en.wikipedia.org/wiki/William_Lee_%28valet%29
George Whitefield is an interesting character in that even when he supported slavery early in life he saw it as a means of gaining freedom from the barbarism of Africa. He later became an abolitionist but always believed that an education for all slaves was essential to their future independence. A slave named Phyllis Wheatly was a well known poet (not a well known black poet) of the day and eulogized Whitefield in glowing terms.
And as a mullato, he was probably half of African descent and half descended from the Lee family.