Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a small bowl, gently mash 1½ cups of blackberries with a fork until juicy but still a bit chunky.
Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
In a large mixing bowl, beat butter and sugar until pale and fluffy. Add the eggs one at a time.
Mix in the vanilla extract, lemon zest, and, if desired, a few drops of purple food coloring.
Alternate adding your dry mixture and buttermilk into the creamed base, starting and ending with the dry ingredients.
With a light hand, fold the mashed blackberries into the batter.
Divide the batter evenly between the pans. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cakes in their pans for 10 minutes, then transfer to wire racks.
Reduce 1 cup of blackberries over low heat until thick, then strain to remove seeds. Beat butter and cream cheese together until smooth, gradually adding powdered sugar. Mix in the cooled blackberry reduction, vanilla extract, and a pinch of salt until the frosting is creamy.
Place the first cake layer on a stand and spread an even layer of frosting. Stack the second layer on top and cover the cake fully. Garnish with fresh berries, mint, and edible flowers.
