“Mary Poppins Returns,” which picked up four Oscar nominations last week, is an enjoyably derivative film that seeks to inspire our nostalgia for the innocent fantasies of childhood, as well as the jolly holidays that the first “Mary Poppins” film conjured for many adult viewers. Part of the new film’s nostalgia, however, is bound up in a blackface performance tradition that persists throughout the Mary Poppins canon, from P. L. Travers’s books to Disney’s 1964 adaptation, with disturbing echoes in the studio’s newest take on the material, “Mary Poppins Returns.” One of the more indelible images from the 1964 film...