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 vegetable oil, buttermilk, eggs, vanilla extract, blackberry puree, and black food coloring (if using) until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then remove from the pans and transfer to a wire rack to cool completely.
For the frosting, beat the softened butter in a large bowl until creamy. Gradually add the powdered sugar, mixing on low speed until combined.
Add the heavy cream, vanilla extract, and blackberry puree to the frosting, and beat on high speed until light and fluffy.
Once the cakes are completely cool, place one layer on a serving plate and spread a layer of frosting on top. Place the second layer on top and frost the top and sides of the cake.
Serve immediately or refrigerate for up to 3 days.