Adjust oven rack to middle position and heat oven to 400°F.
In a large bowl, combine black beans, taco seasoning, salsa, salt, and garlic powder. Stir until evenly mixed and slightly cohesive.
Arrange bell pepper halves, cut side up in a 13x9-inch baking dish. Divide bean mixture evenly among peppers, pressing lightly to fill cavities.
Cover dish tightly with foil and bake until peppers begin to soften, about 20 minutes.
Remove foil, sprinkle with cheese if using, and bake until cheese is melted and peppers are tender, 5–10 minutes longer. Serve hot with a side salad or cilantro-lime rice.
Notes
For softer peppers, cover the baking dish with foil for the first 15 minutes of baking.
Using freshly made salsa will add more moisture and a fresher flavor to the filling.