Remove any debris from the beans, then rinse and soak them overnight in cold water, refrigerated. Drain any remaining water after the soaking, before adding the beans to the soup pot.
In a large soup pot with a heavy bottom, heat the olive oil. Add the ham hock to the oil and begin to brown. Add the onions, peppers, celery, chili, and cumin, and cook over medium heat until the vegetables wilt. Add the garlic and tomato paste and cook an additional 2 minutes, stirring to make sure you don't burn the tomato or garlic. Add the water and bring to a boil.
Add the cleaned and soaked black beans, return to a boil, then lower to a simmer and cook 2 to 2 1/2 hours, or until the beans are tender. Remove the ham hock from the pot, let cool enough to handle, then strip the meat from the bone. Chop the meat coarsely and add back to the soup. Season with salt and pepper to taste. Add the sherry and cook 2 minutes more. Ladle into bowls and garnish each serving with 1 tablespoon sour cream and a sprinkling of scallions or chives.