There's a couple of things I find interesting about Sally Hemings that always gets glossed over in comment threads. First is that Hemings was 3/4 white, and second, that Hemings surviving children were all free as adults.
(William) Beverly and Harriet both ran away from Monticello after their 21st birthday. Jefferson's policy was one of "non-pursuit" for slaves who had reached adulthood and chose to leave. They were not explicitly freed and were not given letters of manumission. They only remained free the rest of their lives because they passed as white and did not disclose details of their origin in their new homes. In the case of Harriet, Jefferson arranged for her to be given $50 and passage.
This solution for slaves that could pass to do so arguably was better than one of manumission. As white people, they faced fewer threats in their live to their freedom than freed slaves did. Freed slaves and blacks born free both, even in the north were periodically captured and re-sold into slavery, as is documented in Solomon Northup's autobiography "Twelve Years a Slave".
The solution that one of my slave ancestors and free seamstress wife came up with was for her to buy his freedom from a plantation owner. They then carefully recorded the bill of sale from this purchase in the country book of sales each time they moved, which was how a genealogist was able to track down their movements (and from his research[1], I connected a known branch of my family via primary sources, which was later re-confirmed by DNA testing). Clearly, this meticulous record-keeping was done as a shield against re-enslavement, although they likely still lived in fear.
This branch of my family crossed the race barrier some time around the civil war, but evidently only somewhat, because rumors followed that family into the 20th century, leading to claims of native American ancestry to explain their darker skin (as it turns out, according to DNA testing, I have ~3% African ancestry but 0% Native American ancestry).
Also, while it is true that freed slaves and their ancestors that crossed the race line faced fewer threats to their freedom than those who didn't, their life was often still full of risk to both life and property since their status was often known to many in the community. One son of the family I mentioned above made good as a trader, and became (the then equivalent of) a multi-millionaire. He married a white woman and lived a long life, and upon his death willed his considerable estate to his son, my 4th g-grandfather (wife had passed).
Unfortunately, one of his wife's nephews moved to annul the will and have the estate passed to him as next of kin, on the grounds that the heir was the product of an invalid marriage, as it was between a black man and white woman. The lower court ruled in favor of the nephew, but it was overruled in one of the first rulings of the reconstruction era SC Supreme Court. Alas, the legitimate heir never managed to recoup much of the estate in spite of the ruling, probably because no one would enforce it.
I'm sure this type of thing was quite common back then, and only grew worse as the region entered the Jim Crow era.
1. http://www.martygrant.com/genealogy/familydata/grpf1652.html
Ironically, I recall my mother's father, now long dead, as being rather virulently racist, but in retrospect it seems likely he's the one my African ancestry came through.
You should look into your ancestry, you may be able to track it down. Just be sure to depend only on primary documents (birth certificates, death certificates, family bibles, wills, census rolls, etc) and perhaps some very convincing secondary sources (like newspaper accounts, or academic works). There's a lot of misleading "genealogy" that people have done in the past 50-75 years, and particularly in the past 10 years in the genealogy forums.