Cut the watermelon into 1/2 inch thick slices, then use a star shaped cookie cutter to cut the slices into stars. Place the watermelon stars in a single layer on a tray, and place in the refrigerator while you make the whipped cream. (The whipped cream will stay on better if the watermelon is cold).
In the bowl of a standing mixer or with an electric mixer, combine the heavy cream and powdered sugar, and beat until light and fluffy, about 5 minutes.
Top each watermelon star with a dollop of the whipped cream, then add blueberries on top of the whipped cream.