In a saucepan, combine sugar and cornstarch;gradually whisk in water and lemon juice until smooth;bring to a boil,stirring constantly. Stir in blueberries; cook and stir for two minutes longer or until thickened and bubbly; remove from the heat and set aside.
In a bowl, combine the dry cake mix and one cup of the oats; cut in six tablespoons butter until crumbly. Set aside one cup of crumb mixture for topping and stir egg into remaining crumb mixture.
Press into a 13" x 9" x 2" baking dish coated with non-fat cooking spray; spread blueberry mixture to within 1/4 inch of edges.
Combine the brown sugar, remaining oats and reserved crumb mixture; cut in remaining butter until crumbly; sprinkle over top of blueberry mixture.
Bake at 350 degrees for 30-35 minutes or until golden brown. These are best served warm (with vanilla frozen yogurt on top!) but if you want the bars to stick together it's best to cool before cutting them. Enjoy!