Preheat the oven to 350 degrees Fahrenheit. Prepare a 9 x 5 loaf pan with parchment paper, leaving an overhang on the 2 long sides. Clip the paper down to the pan using metal binder clips if needed.
In a medium bowl, combine the flour, baking powder and salt and set aside.
In a large bowl, combine the melted and cooled butter with the sugar and whisk well until smooth and combined.
Add the eggs and mix well. Add the vanilla extract.
Mix in the lemon juice, zest and sour cream and mix well until combined.
Add the dry ingredients to the wet ingredients and stir until the flour is just incorporated with still visible streaks of flour.
In another small bowl, toss the sliced blackberries with 1 tablespoon of flour to coat them completely. Gently fold the blackberries into the batter until evenly incorporated and the flour is not visible. Do not overmix.
Pour the batter into the prepared pan. Bake for 48-52 minutes or until a cake tester inserted into the middle comes out with moist crumbs and the top is lightly golden brown. Let the bread cool for at least 10-15 minutes in the pan before removing it using the parchment paper handles. Cool completely before glazing the bread.