Combine refried beans, black beans, scallions, salsa, cumin, chili powder and jalapenos in a medium bowl. Transfer to a shallow 2-quart microwave-safe dish; sprinkle with cheese.
Microwave on High until the cheese is melted and the beans are hot, 3 to 5 minutes.
Spread sour cream evenly over the hot bean mixture, then scatter with lettuce, tomato, avocado and olives (if using).