This is a tried and true blackberry cake my family loves.
1 (18.25 oz) box white cake mix (any brand)
1 quart frozen blackberries
1 can ready made frosting
Cook blackberries until just barely done, covering with a minimum amount of water. If not already sweetened, add sugar to taste. Prepare cake mix as directed on box except replace liquid with the blackberry juice. Add blackberries (reserve 1/2 cup berries plus 1/8 cup juice) and mix well. Bake cake as directed on box. Let cool thoroughly then frost. If making a layer cake, add reserved berries and juice to ready made frosting. If making a bundt cake, remove approximately 1/4-1/2 cup frosting; melt in microwave and mix with blackberries and juice. Drizzle over cake while hot. Let frosting set and cake cool, then drizzle over it again. Can used canned berries if desired.