It was gradually phased out after the formation of the USA. Because of this, there are a lot of very interesting slave records in the rural Pennsylvania counties - I just went through a number of them in Fayette and Cumberland counties. I would have to recheck, but I think that slaves born before 1780 were supposed to be slaves for life, "negro" or "mulatto" births were recorded from about this time, because children born after this date were to get their freedom from the age of 28. Thus, the slaveholders were required to register the births of negro and mulatto children under their authority. They are rather enlightening documents.
https://en.wikipedia.org/wiki/History_of_slavery_in_Pennsylvania
The absolute numbers are not large, yet still significant. What caught my attention was the high proportion of "mulatto" children; it seemed that it was nearly half, although may have been less than this. Then the question is, how much of this was out and out rape, and how much was effectively common law marriage prohibited by law. I suspect a great deal more of the former, but have no way to find out - although I think that it may be possible to discern some through various records.
The more one digs into the history of slavery in the United States, the more one finds one's preconceptions challenged. Society's views have been heavily formed by Hollywood rather than reality, and the laws around slavery hardened substantially in the decades leading up to the Civil War.
Also, relatively speaking, Washington County was fairly well populated by 1800.
My guess on the "st" is that it means "store keeper", although school teacher is hardly out of the question.