Like many exhibitions lately, Mircea Cantor’s felt like a rebus, and a test. Called “White Sugar for Black Days,” it was eclectic in medium, message and emotional pitch. Scattered throughout the gallery’s two rooms was the show’s trademark work, 7 Future Gifts (2008)—a series of concrete ribbons tied around nothing. The invisible gifts ranged from a forlorn little thing placed in a far corner to a towering absence more than 12 feet high. Along with the show’s title, these ribbons, all topped with schematic bows, suggested a warning, perhaps to beware of empty promises. Or, to relinquish the hope that refined sweeteners can brighten our dark times, relieve our real and spiritual hunger.