In a slow cooker, place the chicken breasts and sprinkle them with cumin, garlic salt, chili powder, and black pepper. Pour the diced tomatoes with green chilies over the top. Cover and cook on low for 6-8 hours or high for 4-6 hours, until the chicken is tender.

Cut the tops off the bell peppers and remove the seeds and membranes. Boil in water for about 5 minutes until slightly tender. Remove and set aside to cool.

In a large mixing bowl, combine the cooked Mexican rice, shredded chicken, cheddar cheese, and black beans. Stir until well incorporated.

Spoon the chicken filling into the prepared bell peppers, packing them to the top. Place the cut tops back on the peppers.

Preheat your oven to 350°F. Place the stuffed peppers upright in a baking dish and cover with foil. Bake for 30 minutes, or until the cheese is melted.