Heat the oven and prepare the cake batter as directed on the cake mix box, using the water, oil and egg whites. Pour it into a greased 9x13-inch baking pan and bake for the time on the box (usually 25 to 30 minutes), until a toothpick inserted in the center comes out clean.

Set the pan on a wire rack and let the cake cool for 15 to 20 minutes, until just warm.

Using the round handle of a wooden spoon, poke holes all over the cake about 1 inch apart, going most of the way to the bottom.

Blend the blueberry pie filling in a blender until smooth and pourable, about 30 seconds. Pour it evenly over the cake, then use the back of a spoon or an offset spatula to push it into the holes and spread the rest over the top.

Let the cake cool completely to room temperature (or chill it for 30 minutes), then spread the whipped topping evenly over the blueberry layer.

In a small bowl, stir together the melted butter, brown sugar and crushed graham crackers until crumbly. Sprinkle the crumble over the whipped topping.

Cover and refrigerate for at least 4 hours, or overnight, until the filling is set. Scatter fresh blueberries over the top just before serving, if you like, and cut into squares.