I came up with this when I had a freezer full of blueberries picked by a friend and a houseful of kids who thought they didn't like blueberries. I first tried it with plain white frosting but too much juice saturated the bottom layer and it was just too sweet. The cream cheese frosting really complemented the blueberries much better.
Spread the top of the first layer of spice cake with frosting, just enough so you don't see the cake
Spread the blueberries over the top of the first layer. Frozen blueberries will be very juicy, and if they are too juicy they will saturate the cake, but the flavor will be delightful. Fresh blueberries may need a little help: make sure they are ripe to the point of almost being soft, if they are not at all juicy mash them just a little bit to get the juices flowing.
Put the top layer on the cake
Finish frosting the whole cake
Let the cake absorb the blueberries at least 8 hours -in the fridge