Drizzle chicken breasts with 2 tsp (10 ml) of the oil and sprinkle with salt and pepper. Heat grill pan or nonstick skillet over medium-high heat and cook chicken breasts, turning once, for about 12 minutes or until no longer pink inside. Keep warm.
In another nonstick skillet, heat remaining oil over medium heat and cook onions for about 8 minutes or until softened and golden.
In small bowl, whisk together 1/4 cup (50 ml) of the grape juice and cornstarch; set aside.
Add remaining grape juice and wild blueberries to onion and bring to boil. Stir in cornstarch mixture and cook, stirring for 1 minute or until thickened. Add balsamic glaze, sugar and salt to taste.
Thinly slice grilled chicken breasts and serve with wild blueberry grape sauce and celery for garnish.