Sir William Blathwayt was a powerful official and a patron of William Byrd I and William Byrd II of Westover who acquired his portrait.
Reference: A portrait of “William Blaithwhite” was bequeathed to William Powell Byrd by Mary Willing Byrd
See: Mary Willing Byrd, “The Will of Mrs. Mary Willing Byrd, of Westover, 1813, with a List of the Westover Portraits,” Virginia Magazine of History and Biography 6, no. 4 (April 1899): 345-358; David Meschutt, “William Byrd and His Portrait Collection,” MESDA Journal 14, no. 1 (May 1988): 18-47.