Since Abby Aldrich Rockefeller acquired it in 1935, an 18th century watercolor has stymied scholars, historians and art enthusiasts who hoped to identify its artist. Now, more than 200 years after placing brush to paper, John Rose has been identified as the painter.

Rose’s identity might never have been known if not for the digging of librarian Susan P. Shames. Shames, a decorative arts librarian for the Colonial Williamsburg Foundation, used primary documents and existing scholarship to revisit “The Old Plantation,” which depicts slaves playing music and dancing....