Rinse beans. In a 4- to 5-quart Dutch oven combine beans and the 6 cups water. Bring to boiling; reduce heat. Simmer, uncovered, for 2 minutes. Remove from heat. Cover and let stand for 1 hour. Drain and rinse beans.
In the Dutch oven cook onion, celery, carrots, and garlic in hot oil until tender. Add beans. Stir in cumin, salt, and cayenne pepper. Add broth and the 2 cups water. Bring to boiling; reduce heat. Simmer, covered, for 1 to 1-1/2 hours or until beans are tender.
Using a handheld immersion blender, blend mixture slightly. Return to Dutch oven. Finely shred 1 teaspoon peel from orange; then juice orange. Stir orange juice, peel, and salsa verde into soup; heat through. If desired, top with sour cream. Sprinkle with cilantro and cayenne pepper.