Cut bell peppers in half lengthwise; discard seeds and membranes. Place pepper halves, skin sides down, on a cutting board or work surface; flatten with hand. Place pepper halves on a grill rack coated with cooking spray; grill 12 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 10 minutes. Peel and cut into strips.
Combine bell peppers, 2 tablespoons parsley, 2 tablespoons chives, vinegar, oil, capers, marjoram, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and garlic; toss gently to coat.
Sprinkle fish evenly with remaining 1/2 teaspoon salt and remaining 1/4 teaspoon black pepper. Place fish on grill rack coated with cooking spray; grill 6 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Sprinkle fish with remaining 1 tablespoon parsley and remaining 1 tablespoon chives. Serve with bell pepper mixture.