The Brick Wall: Where Was My Black Ancestor Before 1880?

My maternal second great-grandmother was named Sarah Riddick, born around 1862 in Washington County, N.C. Records show that she worked as a servant for L.B. Davis and his wife, Annie E. Davis. I would like to know if Riddick was a slave and where she came from. The 1880 U.S. census shows she had three children and no