Cut the tops off the bell peppers and remove the seeds and membranes. Place them upright in a baking dish.
In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil, then reduce to low heat, cover, and simmer for about 15 minutes, or until the quinoa is cooked and all the liquid has been absorbed.
In a large bowl, combine the cooked quinoa, black beans, corn, diced tomatoes, cumin, paprika, garlic powder, salt, and pepper. Mix well until all ingredients are evenly combined.
Evenly distribute the quinoa mixture among the bell peppers, packing it down gently. Top each stuffed pepper with shredded cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the peppers are tender and the cheese is golden and bubbly.
Remove from the oven and let them cool slightly before serving. Garnish with fresh cilantro if desired.