Prepare and bake cake mix according to package directions, using 2 9-inch round cake pans. Cool completely.
In a large bowl, dissolve gelatin in boiling water. Add ice water and stir. Place bowl in ice water for about 5 minutes or until slightly thickened. Fold in blackberries and 1/2 cup whipped topping.
Place bottom layer of cake on a large cake server.
Spread half of gelatin mixture on top of bottom cake layer. Then, top with remaining cake layer. Top with additional gelatin mixture. Complete by topping with remaining whipped topping. Garnish with blackberries. Refrigerate leftovers.