“Say it,” implore the posters for the new Candyman sequel, referring to the urban myth that the hook-handed ghost of the title can be summoned by repeating his name five times in front of a mirror. But Nia DaCosta will not say it: “Oh hell, no.” She is shaking her head. “Never have done, never will.” Despite having directed and co-written the film, DaCosta isn’t taking any chances. “In fact, when...