1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom and sides of 9- or 8-inch square pan with baking spray with flour.
2In large bowl, beat cake mix, sour cream, water, oil and egg whites with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Spread batter in pan. Top evenly with rhubarb and blueberries.
3In small bowl, stir topping ingredients until well mixed. Sprinkle evenly over batter and fruit.
4Bake 9-inch pan 30 to 40 minutes, 8-inch pan 35 to 45 minutes or until toothpick inserted in center comes out clean. Cool 20 minutes. Serve warm or cool, Garnish with sweetened whipped cream and fresh mint leaves, if desired. Store loosely covered.