Prepare Blackberry Swirl: In a small saucepan, combine fresh blackberries, granulated sugar, and lemon juice over medium heat for about 10 minutes until thickened. Strain to get blackberry purée.

Preheat Oven: Set your oven to 350°F (177°C) and prepare a 9x5-inch loaf pan by greasing or lining it.

Prepare Dry Ingredients: Sift cake flour and salt into a mixing bowl, then set aside.

Cream Butter and Sugar: Beat softened unsalted butter and granulated sugar together until light and fluffy, then add vanilla extract.

Incorporate Eggs: Add room-temperature eggs one at a time, mixing well after each addition.

Combine Mixtures: Gradually add sifted dry ingredients alternating with sour cream until just combined.

Create Blackberry Mixture: Mix half a cup of the batter with the blackberry purée.

Layer Batters: Pour half of the vanilla batter into the pan, dollop blackberry batter, and top with the remaining vanilla batter. Gently swirl with a knife.

Bake: Bake for 60-70 minutes, check doneness with a toothpick, then cool in the pan for 20 minutes.

Make Icing: Prepare icing by mixing blackberry juice with powdered sugar and vanilla extract until smooth.

Glaze Cake: Pour icing over the cooled cake and let set for 1 hour before slicing.