Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease two 9-inch round cake pans.

In a small saucepan, simmer 1.5 cups of fresh blackberries over medium heat, crushing them until liquid releases. Strain through a fine-mesh sieve to obtain 0.75 cups of seedless puree. Set aside to cool.

In a large bowl, sift together the flour, sugar, baking soda, salt, and cocoa powder.

In a separate bowl, whisk together the vegetable oil, buttermilk, eggs, vinegar, vanilla extract, and 0.5 cup of the cooled blackberry puree.

Gradually incorporate the dry ingredients into the wet ingredients, whisking until just combined and smooth.

Divide the batter evenly between the prepared cake pans and bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.

Cool the cakes in the pans for 10 minutes, then invert onto wire racks to cool completely.

To prepare the frosting, beat the cream cheese and butter together until light and fluffy.

Gradually add the powdered sugar one cup at a time, followed by the remaining 0.25 cup of blackberry puree and a teaspoon of vanilla, beating until the frosting is smooth and holds its shape.

Level the cooled cakes if necessary, then frost the first layer, stack the second layer, and cover the top and sides with the remaining frosting.