Place bell peppers on an aluminum foil-lined baking sheet. Broil 5 inches from heat 15 minutes or until blistered. Place bell peppers in a large zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel peppers, discarding skins.
Process peppers and mango in a blender or food processor until smooth. Add mango nectar and next 6 ingredients; process until smooth*. Cover and chill mixture 2 hours. Top with sour cream, and garnish with fresh mint, if desired.
*Note: If desired, press mango mixture through a wire-mesh strainer, using back of a spoon to squeeze out liquid. Discard pulp and solids. Straining and discarding solids yields less volume but makes a very smooth soup.