Mix rice, black beans, tomatoes, and cooked onion in a large bowl. Add chili powder, garlic salt, cumin, 1/2 teaspoon salt; stir until evenly mixed. Fold 1 1/2 cups Mexican cheese blend into rice mixture. Spoon rice mixture into each bell pepper; arrange peppers in 9×9-inch baking dish. Sprinkle peppers with remaining Mexican cheese blend.