Combine chicken, beans, chickpeas, tomatoes, tomato sauce, onion, broth, Buffalo sauce, oil, oregano and garlic powder in a 5- to 6-quart slow cooker. Cover and cook on High for 4 hours. Remove the chicken and place on a cutting board. Let cool slightly; roughly shred with two forks and return to the slow cooker. Stir in blue cheese and sour cream.