The easiest way to make a blue colour to cake mix is to use blue food dye.
To make a blue cake mix, you'll need to mix together 1 cup of all-purpose flour, 1 cup of sugar, 1 teaspoon of baking powder, and 1/2 teaspoon of salt. Once those ingredients are mixed together, add 1 cup of milk and 2 tablespoons of oil. Finally, add 2 eggs and 1 teaspoon of blue food dye. Stir everything together until everything is well combined, then pour the batter into an oven-safe dish or pan. Bake the cake at 350 degrees Fahrenheit for 25 minutes, or until a toothpick or cake tester comes out clean.