Not only is this my favorite black bean soup recipe EVER, it's also very low in fat! I use the mild version of Rotel, and it adds just a hint of spiciness. You could easily kick-up the heat by using the traditional version of Rotel or by adding more red pepper flakes.
Coat bottom of a 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. Yields about 1 1/2 cups of soup per serving.