Bring a large pot of water and 1 tablespoon salt to a boil; cook green bell peppers in the boiling water until slightly softened, 3 to 4 minutes. Drain.
Heat olive oil in a skillet over medium heat; cook and stir onion in the hot oil until softened and transparent, 5 to 10 minutes.
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 9x9-inch baking dish. Sprinkle peppers with remaining Mexican cheese blend.
Bake in the preheated oven until cheese is melted and bubbling, about 30 minutes.