In a small saucepan, combine fresh blackberries, granulated sugar, and lemon juice over medium heat. Stir occasionally and simmer for about 10 minutes until the mixture thickens. Strain the mixture to remove seeds and allow it to cool.

Preheat your oven to 350°F (177°C) and prepare a loaf pan by greasing it with non-stick spray.

Sift the cake flour and salt together in a medium bowl and set aside.

In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 5 minutes.

Add the vanilla extract and eggs one at a time, mixing thoroughly after each addition. Gradually incorporate the sifted dry ingredients and sour cream until just combined.

Combine about one cup of the batter with the cooled blackberry swirl. Gently stir to create a marbled effect and return it to the main batter, swirling it lightly.

Pour the batter into the prepared loaf pan and bake for 60-70 minutes, or until a skewer inserted into the center comes out clean.

Once baked, cool the cake in the pan for about 20 minutes before transferring it to a wire rack to cool completely.

Prepare optional icing by mixing strained blackberry juice, powdered sugar, milk, and vanilla extract until smooth. Drizzle over the cooled cake.