It sounds like you're talking about Jiffy "biscuit" mix... not "cake" mix, right?

Here's a basic loaf cake recipe that calls for the addition of chopped apples. I don't see why fresh blueberries couldn't be substituted. Hope it's the one!

BRUNCH CAKE

2 cups packaged biscuit mix (Jiffy or Bisquick)
3/4 cup sugar
1 egg
3 tbsp. salad oil
3/4 cup milk
1 cup finely chopped, pared apples

Preheat the oven to 400 degrees F and grease a 9x5x3 loaf pan.

Combine biscuit mix with sugar.

In large bowl with spoon, combine egg with salad oil and milk. Stir in biscuit mix and sugar mixture. Beat 1/2 minute. Fold in apples. Turn into pan.

Bake 35 to 40 minutes.

Serve warm sliced with vanilla ice cream on the side.