You could substitute cranberries in this recipe if you prefer (it will just be a bit more tart than the blueberry version).
- Coat a 9x9x2-inch square baking dish (or similar round pan) with canola cooking spray.
- Add cake flour, whole-wheat pastry flour, 3/4 cup sugar, baking powder, salt, canola oil, 3 tablespoons fat-free sour cream, low-fat milk, and egg to large mixing bowl and beat for about 30 seconds until blended. Carefully stir in blueberries and spread in prepared pan.
- Add the remaining ingredients for crumb topping to a small food processor (or use a pastry blender and a medium-sized bowl) and mix together briefly -- just until blended and crumbly. Sprinkle topping evenly over cake batter.
- Cover dish well and put in refrigerator overnight.
- In the morning, preheat oven to 375°F. Bake in center of oven until cake tests done (about 45 minutes).