Coat bottom of large stockpot with cooking spray. Add onion and garlic and cook, stirring frequently, until onions are soft but not brown, about 5 minutes.
Place one can of beans in blender; add sautéed onion mixture, red pepper flakes and cumin. Cover and blend on high until smooth, about 30 seconds. Pour mixture back into stockpot.
Place second can of beans and broth in blender and puree until smooth; add to stockpot.
Stir third can of beans (do not puree first), tomatoes and green chilies, and corn into stockpot. Bring to a boil, lower heat to medium and simmer for 20 to 25 minutes. Garnish with sour cream. Yields about 1 1/2 cups of soup per serving.