Preheat oven as directed on cake box instructions.
Gently fold 1 cup of blueberries into batter, reserving remaining berries.
Bake as directed on box.
Allow to cool completely on wire rack.
In large mixing bowl, combined remaining ingredients.
Use hand mixer and beat until completely combined.
Allow mixture to gradually thicken.
If making a layer cake, spread some icing on top of bottom layer, then evenly distribute a layer of blue berries.
(If making a layer cake) place top layer on bottom layer.
Apply icing, and top with remaining blueberries.
Chill for an hour or so before serving.
This cake can be made as either a sheet cake or layer cake. Icing this cake is not necessarily a "neat" job! Initially, the "icing" will slide down the sides of the cake (if layered.) Be prepared to have a spoon to retrieve and redistribute whatever icing collects at the base. As you spread it, the icing gradually thickens to become more manageable.