Cook 2 cups of berries with the cornstarch and sugar, then add the other 2 cups and cook until hot and thickened. Let cool.
Preheat oven to 350 degrees.
Spread blueberry filling in the bottom of a 9-inch square pan. Sprinkle with 1 teaspoon lemon juice.
Combine dry cake mix, nuts and melted butter. Mixture will be crumbly. Sprinkle over blueberry filling.
Bake for 40 to 45 minutes until golden brown.
Serve with ice cream, whipped cream, or serve warm with heavy cream.