Purple Poke Cake

1 (18.25 ounce) box white cake mix
1 small box blackberry gelatin
1 cup boiling water
1/2 cup cold water

Prepare cake mix as directed on package, baking in well-greased and floured 13 x 9-inch pan at 350 degrees F for 30 to 35 minutes. Cool in pan 15 minutes.

Poke with fork at 1/2-inch intervals. Dissolve gelatin in boiling water. Add cold water and pour over cake in pan. Chill 3 to 4 hours. Top with thawed Cool Whip?.