Preheat the oven to 350°F. Prep 3 9-inch cake pans by spraying them with nonstick spray. Set aside.
In a small mixing bowl, combine the flour, pudding mix, baking powder, salt, and baking soda.
2½ cups all-purpose flour, 3.4 ounces lemon pudding mix, 2 teaspoons baking powder, 1 teaspoon kosher salt, ½ teaspoon baking soda
In a large mixing bowl, whisk the sugar eggs, sour cream, water, vegetable oil, and lemon juice together. Add in the flour mixture and whisk until the flour is incorporated.
1½ cups granulated sugar, 4 large eggs, ¾ cup sour cream, ½ cup water, ⅓ cup vegetable oil, ¼ cup lemon juice
Evenly divide the batter between the 3 prepped cake pans.
Place into the preheated oven to bake for 18-20 minutes or until a toothpick inserted comes out clean or with moist crumbs. Set aside to cool.
Make the blackberry frosting by adding all of the frosting ingredients to the bowl of a stand mixer fitted with the paddle or whisk attachment. Beat on high for 4 minutes.
16 ounces cream cheese, 1 cup unsalted butter, 1½ cups powdered sugar, 1 cup fresh blackberries, 2 teaspoons pure vanilla extract, ⅛ teaspoon kosher salt
Once the cakes are cooled, remove from the pans. Place one cake layer on a plate or cake stand.
Spread about ¼ cup of the frosting over the cake. Top with a second layer of cake and spread more frosting on it. Top with the last cake layer. Spread the remaining frosting on the top and around the sides of the cake.
Top with lemon slices and blackberries if you so desire. Slice and serve!