Bake the cake using oil, eggs, and water, according to package directions. You can make cupcakes or a 9 x 13 cake. Leave the cake to cool.
Pulse the freeze dried blueberries into a fine powder in a blender.
In a medium bowl, combine butter, cream cheese, blueberry powder, vanilla, lemon juice, and powdered sugar. Add milk to get the frosting to a spreadable texture.
Frost your cooled lemon cake or cupcakes with the blueberry frosting. Decorate with fresh blueberries, if desired.