Place the blueberries in a saucepan over low heat and cook for about 10 to 15min, mixing occasionally.
While the blueberries cook start preparing the batter.
Preheat the oven to 180C degrees. And grease an 8-inch cake pan.
In a big bowl mix together on a low-speed the butter, shortening, and sugar until very light and fluffy, for about 5 minutes. Add the egg whites, one at a time. Allow each egg white to incorporate before adding the next. After adding the last egg white, add the vanilla extract. Mix the batter for about a minute. It should appear very light and fluffy
Reduce speed to low. Add about 1/3 of the flour, followed by the sour cream. Then, alternate between the remaining flour and milk, allowing each to almost incorporate before adding the next. After the last addition of milk, mix the batter until smooth, about a minute.
By this time the blueberries should already be cooked. Sive them and set it aside. That's your natural color for coloring the batter.
Divide the batter among 5 medium bowls. In the first bowl add 4 tablespoons of the sieved blueberries. In the second bowl add 3 tablespoons of the sieved blueberries, in the 3rd bowl add 2 tablespoons of the sieved blueberries, in the 4th bowl add 1 tablespoon of the sieved blueberries and in the last bowl don't add anything.
Bake them in the same order.
While the cakes are baking. Start preparing the frosting