Drain and rinse both cans of black beans, then set the beans aside.
Melt the butter in a 3-quart saucepan, then add the diced onion. Sprinkle onion with a bit of black pepper.
Saute the diced onion for 2 minutes, then add the minced garlic, and continute to saute the onion and garlic for 1 minute more.
Sprinkle the cumin over the onion and garlic, then add the can of refried beans, the salsa, and the beef broth, and stir.
Now add the black beans and corn, and stir. Simmer the soup on low heat, until the soup is heated through, for about 30 minutes.
If desired, serve individual bowls of soup with shredded cheddar cheese, and a dollop of sour cream.
Note: When making this soup, the brand of salsa I used was Red Gold (Medium Heat). The brand of refried beans I used was Old ElPaso Spicy Fat Free Refried Beans. And the brand of whole kernel corn I used was Green Giant Whole Sweet Corn.