Preheat your 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, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Gently fold in the blackberries and lemon juice.
Divide the batter evenly between the prepared cake pans.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.
While the cakes are cooling, prepare the frosting by beating the softened butter until creamy.
Gradually add the powdered sugar and mix until combined.
Slowly add the heavy cream and vanilla extract, beating until the frosting is light and fluffy.
Stir in the blackberry puree until well incorporated.
Once the cakes are completely cooled, place one layer on a serving plate and spread a layer of frosting on top.
Place the second cake layer on top and frost the top and sides of the cake with the remaining frosting.
Garnish with fresh blackberries.
Serve immediately or refrigerate for up to 3 days.