1Heat oven to 375º. Place blackberries in large bowl. Sprinkle with cornstarch; toss until blackberries are coated. Carefully stir in peaches.
2Place fruit in ungreased square baking dish, 8x8x2 inches. Mix remaining ingredients until crumbly; sprinkle over fruit.
3Bake about 30 minutes or until topping is golden brown and fruit is tender.