Step 1Place peppers on wire rack set over gas flame and roast, turning to blacken, until peppers are charred on all sides, 8 to 10 minutes. Or, place under broiler and broil until blistered on all sides. Place in paper bag and let stand 10 minutes until peppers collapse and skin is easy to remove. Peel, seed and chop. Set aside.
Step 2Remove husks and silk from corn. Blanch corn in boiling water 2 minutes. Remove to bowl of ice water to shock. Pat ears dry with paper towels. Cut kernels from cobs and place in mixing bowl. You should have about 8 cups.
Step 3Add beans, peppers, onion and cilantro to corn and toss.
Step 4Combine oil, lime juice, cumin, garlic and salt and pour over corn mixture, tossing to coat. Cover in serving bowl and chill several hours or up to 2 days for flavors to blend. When ready to serve, sprinkle with queso fresco.