For the cake: Preheat the oven to 350 degrees F. Butter and flour two 9-inch-diameter cake pans with 1 1/2-inch high sides.
Sift the flour, cocoa powder, baking powder, baking soda and salt into a medium bowl. Whisk the buttermilk, food coloring, vanilla and vinegar in a small bowl to blend. Using an electric mixer, beat the granulated sugar and butter in a large bowl until well blended. Add the eggs one at a time, beating until well blended after each addition. Beat in the dry ingredients in four additions alternately with the buttermilk mixture in three additions.
Divide the batter between the prepared pans. Bake until a tester inserted into the center of the cakes comes out clean, about 25 minutes. Cool in the pans on racks, 10 minutes. Turn the cakes out onto racks to cool completely.
For the frosting: Beat the butter and cream cheese in a large bowl until smooth. Beat in the vanilla. Add the confectioners' sugar and beat until smooth.
To assemble: Place one cake layer, flat-side up, on a platter. Spread 1 cup frosting over the top of the cake. Arrange 1 pint raspberries and 1/2 pint blueberries atop the frosting, pressing lightly to adhere. Top with the second cake layer, flat-side down. Spread the remaining frosting over the top and sides of the cake. Arrange the remaining berries decoratively over top of the cake.