Weight Watchers Lemon Cake

1 box lemon cake mix
2 egg whites
10oz Diet 7UP
1 cup blueberries

Preheat oven to 350 degrees.  Whisk together cake mix, egg whites and Diet 7UP. Fold in blueberries.  Pour cake into a greased 9x13 pan and bake for 30 minutes.  Top with cool-whip.