Place first 4 ingredients in a food processor or blender. Pulse 2 to 3 times or until blended. With processor on, slowly pour olive oil through food chute, processing until blended. Set dressing aside. Warm tostada shells according to package directions.
Place 1 tostada shell on each plate; top each with 1/2 cup shredded lettuce, 1/4 cup black beans, 1/3 cup corn, 1/4 cup tomato, 1 tablespoon onion, and 2 tablespoons bell pepper. Spoon dressing evenly over salads. Serve with sour cream, if desired.