I took a look at the LVA catalog to see what they had, possibly a Free Negro Register for Prince William County, but they don't appear to. Although not all counties kept separate registers and information on Free Blacks could and did appear in the county court order books.

You might want to contact the RELIC (Ruth E. Lloyd Information Center for Genealogy and Local History) at the Prince William County Library. It appears that the library site in which they are located is under renovation, and they have temporarily moved to another branch, but they could tell you what they have, or where you could look.