Sort the beans carefully to remove any dirt and debris. Rinse the beans in cold water and then soak them overnight covered with more, clean, fresh cold water and place in the refrigerator. Drain any excess water before adding the beans to the soup pot.
In a large soup pot with a heavy bottom, heat the olive oil. Add the onions, peppers, celery, chili powder, and cumin, and saute over medium heat until the vegetables wilt. Add the garlic and tomato paste and cook an additional 3 minutes, stirring to make sure you don't burn the tomato or garlic. Add the water and bring to a boil.
Add the black beans, return to a boil, then lower to a simmer and cook for 2 to 21/2 hours, or until the beans are tender. Remove from the heat, season with salt and pepper to taste, and allow to cool for half an hour before pureeing in small batches in a blender or food processor.
If you are serving immediately, return the soup to a boil and add 1/4 cup dry sherry. Cook 2 minutes more. Ladle into bowls and garnish each with a tablespoon of sour cream and a sprinkling of scallions or chives. If you are preparing the soup ahead of time, cool thoroughly after blending, transfer to a container with a tightly fitting lid, and store in the refrigerator for up to a week. Add the sherry after reheating the soup to serve.