HEAT grill to medium heat.
HEAT 1/4 cup dressing in large skillet on medium-high heat. Add onions; cook and stir 2 to 3 min. or until crisp-tender. Add peaches; cook 2 to 3 min. until heated through, stirring occasionally. Stir in blueberries. Remove from heat; cover to keep warm.
BRUSH chicken with remaining dressing. Grill 5 min. on each side. Brush with barbecue sauce; grill 4 min. or until done (165F), turning and brushing occasionally with remaining barbecue sauce.
SERVE chicken topped with fruit mixture.