The noisy chatter and clinking of glasses that is the soundtrack of successful restaurants was absent on a recent weeknight at Mercat Negre. The Spanish small-plates eatery, a Williamsburg spin-off of Manhattan’s Mercat, was empty—a bad omen just a few months after opening. It’s certainly not the attractive decor keeping the hordes away: Subway tiles wrap one wall; a steel counter overlooking an open kitchen is tended to by Iberian chef Oriol Sala Colomer, a protg of Ferran Adria of El Bulli. That kind of pedigree had us hoping for some culinary sorcery, but the drab food was lacking in both vision and execution. Patatas bravas—that old tapas standby of fried potatoes—were tasty enough, with alternating stripes of hot sauce and crema, and a duck magret was a gorgeously pink cube of meat, served in a puddle of its own savory stock. But other dishes flopped. Confit chicken wings speckled with black sesame seeds were greasy and anemic, with flaccid skin over rubbery meat. The fideus caador (Valencia’s answer to paella, made with short noodles) featured dull hunks of rabbit, chicken and pork, and lacked the slight crispiness that typically defines the dish. For dessert, we tried the pa amb xocolata—a fancy name for thin, olive-oil-rubbed toast topped with chocolate and sea salt. It would have been a lovely, simple finale—if we weren’t so unsatisfied by the meal that preceded it. Mercat Negre’s saving grace might be its rooftop space, built out with wooden planks like a suburban porch. In warmer months, we can imagine savoring a glass of Navarrese grenache ($7) outdoors. But at this rate, we wonder if the restaurant can hold on till summer.