In a saucepan, combine 4 ounces of blackberries, 3 tablespoons of sugar, and 1 tablespoon of lemon juice. Heat over medium until the mixture reduces to about 1/3 cup, approximately 10 minutes. Press through a sieve to remove seeds and set aside.

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

Sift together the cake flour and salt, then set aside.

In a mixing bowl, cream the softened butter with 1 and 1/2 cups of sugar until fluffy, about 5 minutes. Mix in the vanilla extract.

Add eggs one at a time, mixing thoroughly after each addition.

Gradually incorporate the dry ingredients in three parts, alternating with sour cream in two parts.

Take 1/2 cup of the batter and mix it with the reserved blackberry swirl.

Pour half of the pound cake batter into the prepared pan, followed by the blackberry mixture, and top with the remaining batter. Swirl carefully with a knife.

Bake the cake for 60-70 minutes, or until a toothpick comes out with moist crumbs.

Allow the cake to cool in the pan for 20 minutes before turning it out to cool completely.

To make the icing, press 2 ounces of blackberries through a sieve to extract juice. Combine the juice with 1 cup of powdered sugar, 1 tablespoon of milk, 1/2 teaspoon of vanilla extract, and 1/8 teaspoon of salt.

Pour the icing over the cooled pound cake and let set for about 1 hour.