1. Sprinkle chicken breasts with salt and pepper.
2. Coat food rack with cooking spray; place on grill over medium-high heat (350° to 400°). Place chicken on rack, and grill 10 minutes on each side or until done.
3. Shred chicken, and place in a large bowl. Stir in 1 can tomatoes with green chiles, half of black beans, and 1 cup of cheese.
4. Spoon chicken mixture evenly down center of each tortilla; roll up tortillas, and place, seam sides down, in a 13- x 9-inch baking dish. Top with enchilada sauce and remaining tomatoes with green chiles, black beans, and cheese.
5. Bake at 350° for 30 minutes. Remove from oven, and let stand 10 minutes. Top each serving with avocado slices and sour cream.