In a medium mixing bowl combine the flour, baking powder and salt; mix. Set aside.
In a large mixing bowl (or bowl of a standing mixer) add the softened butter; mix well until light and fluffy.
Add in the 1 cup of granulated sugar and mix well.
Add the eggs one at a time, combining after each addition.
Add the sour cream, lemon juice, lemon zest and vanilla extract. Mix just until just combined.
Add in the flour mixture; mix just until combined.
Spoon half of the batter into the prepared loaf pan. Top with evenly half of the blueberry mixture. Using a toothpick or butter knife swirl the blueberries into the top of the batter.
Spoon in the remaining batter and repeat the swirl on top of the loaf.
Place into the oven for 50-60 minutes or until a toothpick comes out clean. NOTE: If the top begins to brown before the inside is cooked cover with some foil, dull side facing the pound cake.
Remove the cake and set on a cooling rack for 10 minutes. Remove from the loaf pan and place on a cooling rack with a rimmed baking sheet underneath. Allow to fully cool.