Snap off the chile stems, then roast the poblanos over a gas flame or under a broiler until the skins are lightly charred all over. Place the poblanos in a paper or plastic bag, cover, and let them steam for 15 to 20 minutes. Peel away the skin, and seed. Dice the peppers, then combine in a large bowl with the black beans, corn, watercress, tomato, and scallions. In a small bowl, whisk together the vinaigrette ingredients. Pour over the salad and toss to coat. Serve chilled or at room temperature.

drain them.