Preheat oven to 350 degrees F (or 325 degrees F for dark-coated metal pan). Position oven rack in middle of oven. Spray the bottom only of a 13 x 9-inch metal baking pan with nonstick cooking spray (or foil-line pan).
Set aside 3 tablespoons of the cake mix. Place the remaining cake mix and melted butter in a large bowl. Blend for 1-2 minutes with an electric mixer set on low speed until smooth. Stir in oats until blended. Press half of mixture evenly into prepared pan.
Stir together the blueberries, jam, reserved 3 tablespoons cake mix, and lemon zest in a large mixing bowl. Spoon and spread berry mixture over prepared crust; top with reserved crumb mixture and press down gently.
Bake for 30-34 minutes or until topping is set and firm to the touch. Transfer to wire rack and cool completely.