Preheat oven to 350°F. Grease and flour 3 (8-inch) cake pans. Set aside.
Using an electric mixer, beat the butter and sugar together until light and fluffy. Add the eggs, one at a time, mixing just until the yolk disappears.
In a small bowl whisk together the flour, baking powder, baking soda and salt. Set aside.
In another small bowl, blend the sour cream, milk and vanilla together. Set aside.
Add 1/2 of the sour cream mixture to the sugar mixture and blend on low until incorporated. Next add 1/2 of the flour mixture and mix on low until blended. Scrape down the sides and bottom of the bowl as needed. Add the remaining sour cream and blend, then all the remaining flour. Blend on low just until incorporated. Scrape down the sides and bottom of the mixing bowl with a spatula. Add the lemon juice and zest. Using the spatula, gently fold the lemon juice and zest into the batter. Do not beat!
Divide the batter equally between the three prepared 8-inch cake pans and bake until a toothpick inserted in the center comes out clean, about 20-25 minutes, rotating the pans after 15 minutes. Do not over bake!!! Cool in the pans for 10 minutes, then turn the cakes out and cool completely on a wire rack.
To prepare the filling:
Gently warm the blueberry jam in a small bowl in the microwave at 50% power for 20 seconds; stir and repeat until slightly melted. Set aside 2 heaping tablespoons of the warm jam liquid to color the frosting. Fold 2 cups of fresh blueberries into the remaining warm jam. Microwave the blueberry mixture at 50% power for 30 seconds. Stir and repeat until the blueberries are slightly soft but not ruptured, about 2 minutes total. Set aside to cool while preparing the frosting.
To prepare the frosting:
Combine the cream cheese and butter and beat until blended. Add the lemon juice, vanilla, powdered sugar and salt. Beat until fluffy.
To assemble the cake:
Place one cake layer on a cake stand or plate. Spread 1/4 of the frosting on top of the cake leaving the icing higher all the way around the outside edge of the circle. This will help hold in the blueberry filling. Spoon half the cooled blueberry filling onto the frosted cake layer. Repeat with another layer of cake, 1/3 of the remaining frosting and then the remaining blueberry filling. Top with the last cake layer. Frost the top of the cake and then around the top layer. Add 1 tablespoon of the reserved blueberry jam liquid to the remaining frosting. Frost around the middle layer of the cake. Add the final tablespoon of jam to the last of the frosting, then spread around the bottom layer of the cake to achieve a light ombre affect.
Refrigerate until ready to serve.
Garnish with additional blueberries, or edible flowers if desired.
I DO NOT recommend using frozen blueberries in the filling.