Preheat oven to broil. Place oven rack 3 inches from heat source. Coat baking sheet with cooking spray, and spread tomatillos, onion, red bell pepper, and garlic on prepared baking sheet. Broil 12 minutes, or until tomatillos are blistered and charred, and onion and bell pepper are softened and charred, tossing once halfway through cooking time.
Pulse garlic, 1 Tbs. cilantro, jalapeño, and tomatillos 10 times in food processor, or until chunky. Season with salt and pepper.
Toss 1/4 cup tomatillo sauce with black beans, red bell pepper, and onion.
Fill tortillas with black bean mixture, avocado, and queso fresco. Garnish with remaining cilantro and corn kernels, if desired. Serve remaining tomatillo sauce and sour cream, if desired, on side.