Lightly coat the inside of a 6-qt slow cooker with nonstick spray.
In a large bowl, combine beef, rice, 1 cup cheese, black beans, corn, salsa, cilantro, cumin and chili powder; season with salt and pepper, to taste. Spoon the filling into each bell pepper cavity.
Place peppers into the slow cooker. Cover and cook on low heat for 5-6 hours or high for 2-3 hours, or until the peppers are tender and the beef is cooked through.
Uncover and top with remaining 1/2 cup cheese. Cover and cook on low heat for an additional 10-15 minutes, or until the cheese has melted.
Serve immediately, drizzled with sour cream, if desired.