If using a cake mix, prepare the cake according to package directions and let the cake cool on wire rack.
In a large saucepan, stir together the sugar and cornstarch. Stir in the water.
Add the blackberries. Cook and stir till mixture is thickened and bubbly. Cook and stir 2 minutes more. Remove the saucepan from the heat. Transfer the mixture to a bowl. Cover and chill.
Serve the slices of cake topped with some of the fresh berries, blackberry sauce and whipped cream, if you like. Makes 10 servings.