Roast the chiles over an open flame or broil them, turning with tongs until charred all over, about 10 minutes. When the chiles are cool enough to handle, pull off the skin. Remove the stems and seeds. Cut the chiles into 1/4-inch dice and transfer to a large bowl.
In a large frying pan, heat 4 tablespoons of the oil over moderate heat. Add the corn kernels, 1/2 teaspoon of the salt and 1/4 teaspoon black pepper. Sauté the corn kernels, stirring occasionally, until just cooked, about 5 minutes. Add them to the roasted chiles.
Cut the avocado into 1/4-inch dice and add to the corn with the bell pepper, scallions, vinegar, the remaining 2 tablespoons oil and 1/4 teaspoon salt, and 1/8 teaspoon black pepper. Stir and let sit at room temperature for 25 minutes so that the flavors blend.