Cook onion and cumin in oil in 4-quart saucepot over medium heat until onion is tender, about 3 minutes. Remove from heat. Puree 2 cans of black beans and their liquid in batches with chicken broth in a blender; add to pot. Stir in remaining beans, salsa, lime juice and peel. Heat mixture to boiling; reduce heat to low. Simmer for 30 minutes. Ladle 1 cup soup into a bowl and top with a dollop of yogurt or sour cream.