1. Preheat oven to 350°. Grease 2 (9-inch) round cake pans, and dust with cocoa. Set aside.
2. Beat cake mix and next 11 ingredients at low speed with an electric mixer 1 minute; beat at medium speed 2 minutes. Fold in chopped chocolate. Pour batter into prepared pans.
3. Bake at 350° for 30 to 32 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on a wire racks 10 minutes. Remove from pans to wire racks, and cool completely. Wrap and chill cake layers at least 1 hour or up to 24 hours.
4. Using a serrated knife, slice cake layers in half horizontally to make 4 layers. Place 1 layer, cut side up, on cake plate. Spread one-third of blackberry filling over cake. Repeat procedure twice. Place final cake layer on top of cake, cut side down. Spread chocolate fudge frosting on top and sides of cake. Drizzle remaining filling over top of cake, letting it drip down sides of cake. Cover and chill in refrigerator until ready to serve. Just before serving, garnish, if desired.
As a general rule in cake baking, grease cake pans with shortening unless the recipe states otherwise.