Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.
In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Carefully stir in the boiling water until the batter is smooth. Gently fold in the blackberries.
Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
While the cake cools, whip the heavy cream with the powdered sugar until soft peaks form.
Once the cakes are completely cool, place one layer on a serving plate. Spread a layer of whipped cream on top, then place the second cake layer on top. Frost the top and sides with the remaining whipped cream. Garnish with additional blackberries.