Preheat your oven to 350°F (175°C). Grease and flour three 6-inch pans for even baking. Toss the blueberries and lemon zest with a little flour to prevent them from sinking.
In a large bowl, sift together flour, baking powder, salt, and sugar. Whisk them until combined to ensure even distribution for a light and fluffy cake texture.
In another bowl, whisk milk, sour cream, egg whites, lemon juice, zest, and vanilla. Set aside while you cream the butter to save time.
Beat softened butter until light and fluffy. Gradually mix in dry ingredients, then slowly add wet ingredients. Fold in the blueberry mixture carefully to keep berries intact.
Divide batter evenly into pans and bake 25–30 minutes, or until a toothpick inserted comes out clean. Let cakes cool on wire racks before assembling.
Make lemon buttercream and blueberry buttercream separately. Mix part of the buttercream with blueberry reduction for a vibrant flavor. Use lemon buttercream at the edges and blueberry in the center for layered frosting.
Layer cakes with buttercream, smoothing the exterior with a spatula. Decorate with piped dollops of buttercream and top with frozen or fresh berries for a visually appealing finish.