Make the Blackberry Swirl: In a small saucepan, combine fresh blackberries, granulated sugar, and lemon juice. Over medium heat, simmer for about 10-12 minutes until thickened. Strain through a sieve to remove seeds and cool.

Prepare to Bake: Preheat oven to 350°F (177°C) and prepare a 9x5-inch loaf pan with non-stick spray or parchment paper.

Combine Dry Ingredients: In a medium bowl, sift together cake flour and salt until well mixed. Set aside.

Cream Butter and Sugar: In a stand mixer, cream unsalted butter and granulated sugar on medium speed until light and fluffy, about 3-5 minutes.

Incorporate Eggs: Add eggs one at a time, beating well after each addition, about 1-2 minutes.

Mix in Dry Ingredients and Sour Cream: Alternately add sifted flour mixture and sour cream to the batter until just combined.

Create the Blackberry Batter: Mix a portion of the batter with cooled blackberry purée.

Layer the Batter in the Pan: Alternate layers of plain batter and blackberry batter in the loaf pan, swirling gently.

Bake the Pound Cake: Bake for 60-70 minutes, checking for doneness with a toothpick.

Cool the Cake: Allow to cool in the pan for about 20 minutes before transferring to a wire rack.

Prepare the Icing: Mix blackberry purée with powdered sugar, milk, and vanilla until pourable.

Ice the Cake: Drizzle icing over the cooled cake and allow it to set before slicing.