Preheat the barbecue to medium-high, turning on just one burner.
In a bowl, combine the dressing ingredients. Set aside and keep cool.
In a saucepan of boiling salted water, cook the corncobs for 5 to 6 minutes. Drain.
In a bowl, mix together the peppers and the zucchini, onion and olive oil. Add salt and pepper.
Cut off excess fat around the duck breasts. Make incisions in the skin in a crosswise pattern, without piercing the meat. Add salt and pepper.
On a hot and oiled barbecue grill, place the duck breasts on the side with the lit burner, skin side down. Grill for 1 minute 30 seconds, turning breasts halfway through cooking.
Transfer the breasts to the side of the grill with the burner off for indirect cooking, skin side down. Close the lid and continue cooking for 8 to 10 minutes.
Transfer the duck breasts back to the grill over the lit burner, skin side up. Grill for 2 minutes with the lid open.
Place the duck breasts on a cutting board. Cover loosely with a sheet of aluminum foil. Let stand for 3 to 4 minutes before slicing to give the fibers of the meat time to soften.
Meanwhile, grill the corncobs, peppers, zucchini and onion for 3 to 4 minutes on the grill, turning over a few times.
Remove the kernels from the cob using a knife.
Divide the romaine lettuce among four bowls. Garnish with the grilled vegetables, avocado, black beans and slices of duck meat. Drizzle the dressing on top. If desired, top with cilantro.
