In a saucepan, combine 1 cup fresh blackberries, 1/4 cup granulated sugar, and 1 tablespoon lemon juice. Cook over medium heat for about 10 minutes, stirring occasionally, until thickened. Strain the mixture through a fine sieve to remove seeds.

Preheat oven to 350°F (177°C) and spray a 9x5 inch loaf pan with non-stick spray.

Sift together 2 cups cake flour and 1/2 teaspoon salt in a mixing bowl.

In another bowl, cream 1 cup softened unsalted butter and 1 3/4 cups granulated sugar until light and fluffy, about 5 minutes.

Gradually add 4 room-temperature eggs one at a time, mixing well after each. Stir in 1 teaspoon vanilla extract.

Incorporate dry ingredients by alternating with 1 cup sour cream until a smooth batter forms.

Mix 1/2 cup of batter with blackberry puree until smooth.

Pour half of the pound cake batter into the loaf pan, then add the blackberry mixture. Top with remaining batter, swirling gently.

Bake in the preheated oven for 60-70 minutes. Check for doneness with a toothpick. Let cool in the pan for 20 minutes before transferring to a wire rack.