Preheat oven to 350 F. In electric mixer combine crumb topping ingredients and mix until crumbly about 3-5 minutes. You should see pea-sized pieces. Do not over-mix or mixture will become a solid mass. Set aside. In small bowl combine sugar and cornstarch until well mixed. In separate mixing bowl, combine grapes and blackberries. Sprinkle sugar and cornstarch over fruit and toss to coat. Place into 8-inch square baking dish. Sprinkle crumb topping over fruit, coating evenly. Place pan in oven and bake for 40-45 minutes or until top is golden brown and fruit is bubbling around edges.