Lightly coat the inside of a 6 quart slow cooker with nonstick spray. Cut tops off bell peppers, remove stems and seeds.
In a large bowl, combine beef, barley, 1 cup cheese, black beans, corn, salsa, cilantro, cumin, and chili powder.
Spoon filling into each bell pepper cavity until full. Place peppers in slow cooker.
Cover and cook on low heat for 3-4 hours or high heat for 2-3 hours, or until peppers are tender and beef is cooked through.
Uncover and top with remaining 1/2 cup cheese when 30 minutes is remaining to melt.
Serve immediately. Drizzle with sour cream or salsa as desired.