More than a generation ago, Anthony Blunt and Denis Mahon developed ways of thinking about Nicholas Poussin and his art that, although recently the subject of prolonged scrutiny and occasional criticism, still remain canonical. Poussin, the French-born philosopher-painter, returned to his native country as an adult only briefly, when commanded by Cardinal Richelieu to organize the renovation of the Grande Galerie of the Louvre. This great painter was by choice a lifelong resident of Rome—and...