From the Loudoun Co Personal Property Tax Lists:
Thornton, Samuel 1 negro male >21y 1 negro male >16y 1866 - 2nd Dist of Hiram N. Tavenner
Thornton, Samuel 1 negro male >21y residence or employee of Catharine Freer 1867 - 2nd Dist of H. N. Tavenner
I do not find a Catharine Freer in Loudoun during that time, but on the 1860 census there is an Elizabeth Freer with probably daughters Elizabeth, Mary, Charlott, and Octavia. Catharine was from England and the first three daughters were born there, while Octavia was born in the District of Columbia.