Bake cake in 13x9 pan according to package instructions. Allow to cool.

Using a wooden skewer or the end of a wooden spoon, poke holes in the top of the cake.

In a blender, liquify the blueberry pie filling. Then pour the blueberry filling over the cake, using a spoon to work it into the holes.

Top the blueberry filling with whipped cream.

Melt butter and mix together with brown sugar and crushed graham crackers. Sprinkle this over the top of the poke cake.

Chill for 4 hours before serving.

When it's time to serve, top with additional blueberries, if desired.