To make the cake: Preheat oven to 325 F (160C). Spray a 9×5-inch loaf pan with nonstick cooking spray.
In a medium bowl, sift together the flour, baking powder, and salt. Whisk the sour cream and lemon juice together in a small measuring cup.
In the bowl of a stand mixer fitted with the paddle attachment, combine the lemon zest and sugar. Rub together with your fingertips until the sugar is moist and fragrant. Add the butter to the bowl and beat the butter/sugar mixture on medium speed until very pale and fluffy, about 3-4 minutes. Scrape down the sides of the bowl. Mix in the eggs in 3 additions, beating well after each addition until the mixture is smooth. With the mixer on low, alternately add the dry ingredients in 3 additions and the sour cream mixture in 2 additions, starting and ending with the dry ingredients. Once all the ingredients have been added, beat on low for another 30 seconds to fully combine. At the very end, mix in the blueberries.
Transfer the batter to the prepared pan. Smooth the top then tap the pan on your work surface to release air bubbles. Bake the cake for 55-65 minutes, or until a thin knife inserted near the center comes out with just a few moist crumbs. Transfer the pan to a wire rack and let the cake cool for 10 minutes, then turn it out.
Serve cake with fresh blackberries or blueberries and dust with confectioner's sugar! Enjoy.