Scion of a prominent Virginia family, Blair was born at Williamsburg in 1732. He was the son of John Blair, a colonial official and nephew of James Blair, founder and first president of the College of William and Mary. Signer Blair graduated from that institution and studied law at London's Middle Temple. Thereafter, he practiced at Williamsburg. In the years 1766-70 he sat in the Virginia House of Burgesses as the representative of William and Mary. From 1770 to 1775 he held the position of clerk of the colony's council.