Preheat the oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.
Add eggs one at a time, mixing well after each addition. Stir in vanilla extract, lemon zest, and lemon juice.
Alternate adding the dry ingredients and buttermilk to the wet mixture, beginning and ending with the dry ingredients. Mix just until combined.
Gently fold in the blackberries, which have been lightly coated in flour to prevent sinking.
Divide the batter evenly between the prepared cake pans and smooth the tops with a spatula.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
For the frosting, beat the softened cream cheese and butter together until smooth.
Gradually add powdered sugar, mashed blackberry juice, lemon zest, and vanilla extract. Beat until light and fluffy.
Place one cake layer on a serving plate and spread a generous amount of frosting on top. Repeat with the second layer.
Cover the entire cake with frosting, smoothing it with a spatula or offset knife.
Decorate with fresh blackberries, lemon slices, mint leaves, or white chocolate shavings as desired.
Chill the cake in the refrigerator for 30 minutes before slicing. Let it sit at room temperature for 10 minutes before serving.