Place the two chicken breasts at the bottom of your slow cooker. Sprinkle with cumin, garlic salt, chili powder, and black pepper. Pour the can of diced tomatoes with green chilies over the chicken, ensuring it's well coated. Cover the slow cooker, set it to low, and cook for 6-8 hours or on high for 4-6 hours until the chicken is tender and easily shredded.

Bring a large pot of water to a boil. While waiting, rinse the bell peppers, slice the tops off, and carefully hollow them out, removing all seeds and white parts. Set the prepared peppers aside as you wait for the water to boil.

Once your pot of water is boiling, gently place the hollowed bell peppers and their tops into the water. Allow them to cook for about 5 minutes until they start to soften. Remove the peppers from the water, letting them cool upside down on a plate to release excess moisture.

In a large mixing bowl, combine the cooked Mexican rice, shredded chicken, shredded cheddar cheese, and rinsed black beans. Stir together until everything is evenly mixed.

Carefully fill each bell pepper with the delicious chicken and rice mixture, packing it in generously. Optionally, sprinkle more shredded cheddar on top before replacing the pepper tops.

Preheat your oven to 350°F (175°C). Place the stuffed peppers upright in a baking dish and cover with aluminum foil. Bake for 30 minutes, or until the cheese is melted and bubbly, and the peppers have reached your desired tenderness.