In a large skillet, over medium heat, combine the black beans, cooked rice (if using), cumin, garlic powder, chili powder, salt, and pepper. Cook for about 5 minutes, stirring occasionally, until heated through. Remove from heat.
Lay out the flour tortillas on a flat surface. Divide the bean mixture evenly among the tortillas (about ½ cup per tortilla).
Sprinkle an equal amount of cheddar and Monterey Jack cheese over the bean mixture on each tortilla.
Fold the sides of each tortilla towards the center, then roll from the bottom up to form a burrito. Make sure to tuck in the sides as you roll to secure the filling.
In the same skillet, place the rolled burritos seam side down and cook over medium heat for about 3-4 minutes on each side or until the tortillas are golden and crisp, and the cheese is melted.
Once done, remove the burritos from the skillet and let them sit for a minute before cutting them in half.
Serve warm, garnished with fresh cilantro, and with salsa or hot sauce on the side if desired.