Preheat oven to 400. Combine black and refried beans in a saucepan, and add the chili powder, cumin and garlic. Simmer for 5 minutes. Mash the bean mixture into a thick puree (I use a potato masher right in the pan).
On a plate, lay a tortilla flat and place 2 tablespoons of the bean mixture in the center of the tortilla. Top with some scallions, tomatoes and cheese. Roll up into a tube shape and place into a 9x13-inch baking pan, seam side down. Repeat until you've used all the tortillas and/or bean mixture.
Bake the burritos until they are browned, about 15 minutes. Top with salsa and/or sour cream as desired.