A scapegoat for the royalist excesses of 1814, Blacas was unofficially exiled as the French ambassador to the court of the Kingdom of Two Sicilies, whose capital was located in Naples. There, he negotiated the 1816 marriage of the Louis XVIII's nephew, the Duke of Berry, to Francis I of the Two Sicilies's daughter Caroline. Also in 1816, Blacas became a member of both the Académie des Inscriptions et Belles-Lettres and the Académie des Beaux-Arts. Afterwards, he was appointed to be the French ambassador to the Holy See in Rome. He signed a concordat between Bourbon France and Pope Pius VII on 11 June 1817. In 1820, he received the "Ordre du Saint-Esprit". While still ambassador in Rome, he was one of three French representatives to the Congress of Laibach in 1821.