The sleek, clean-lined bar, which spills onto a patio and reflecting pool in warm weather, sits adjacent to Jeff and Barbara Black’s upscale American dining room. The $6 martinis flow, and the crowd is made up of mostly moneyed suburbanites and young professionals. It doesn’t, in other words, look like the kind of place where you’d get a 50-cent spiced-shrimp deal. But weekdays between 4 and 7, that’s exactly what you’ll find along with half-price mussels and discounted cocktails. If you’re hungry, it’s one of the best happy hours around.