Preheat oven to 350 degrees. Coat a muffin pan with cooking spray with flour, see shopping tip. In an electric mixer or large bowl using a hand mixer, add cake mix, flour, pudding, buttermilk, water, applesauce, canola oil and lemon juice. Mix until blended. Add egg whites, egg and beat until smooth. By hand, gently fold in blueberries.
Pour batter into the muffin cups. Fill each to almost the top. Spread evenly.
To make streusel: In a small bowl, add oats, sugar, flour, cinnamon and mix together with a fork. Gradually add melted butter and stir to coat well. Top each muffin with about 1 tablespoon streusel.
Bake for 30-35 minutes until a toothpick inserted comes out clean. Remove from oven and cool for about 10 minutes. Carefully loosen each muffin with a knife and remove onto a plate. Cool before adding the drizzle.
To make the drizzle: In a small dish, combine sugar, lemon juice and mix until smooth.
Drizzle 1 teaspoon over the top of each muffin. Store muffins covered on the kitchen counter for several days. They freeze great too.