In a medium-size stock pot, add dried black beans and 6 cups of water, cover and let soak overnight.
In another large stock pot, heat olive oil and add onion, minced garlic and chopped celery. Saute until vegetables are softened.
Drain and rinse soaked black beans. Add pre-soaked beans or drained and rinsed canned beans to vegetable mixture along with 2 cups water and broth. Bring to boil; reduce heat and simmer.
Add cayenne pepper and ground cumin. Partially cover the pot and simmer over low heat for 2 to 2 1/2 hours, or until beans are soft.
Puree soup in batches in food processor or blender. Return pureed soup to stock pot and simmer.
Add vinegar, sherry, soy sauce and pepper. Serve hot with a dollop of sour cream or yogurt and chopped green onions.
