Preheat oven to 325°F. Grease a 10-inch (12 cup) bundt pan with butter, lightly coat with flour, and set aside.
Whisk together flour, salt, and baking soda in a medium bowl. Combine butter and sugar in the bowl of a stand mixer fitted with a paddle attachment. Cream on medium low speed, scraping down bowl as needed, until light and fluffy, 4-5 minutes. Beat in eggs, one at a time, until fully combined. Scrape down sides of bowl, beat in vanilla.
Reduce speed to low. Alternate adding the dry ingredients and buttermilk in 3 additions, starting and ending with the dry ingredients. Mix in lemon juice.
Spoon a third of the batter into the prepared bundt pan. Tap the pan firmly against the counter to get rid of air bubbles. Using the back of a spoon, create a shallow trench in the center of the batter. Fill the trench with half of the preserves, making sure to keep a small, clear border just along the edges of the pan (this will prevent the jam from leaking out and sticking). Gently press half the blackberries and blueberries into the jam. Spoon another third of batter on top. Make another trench in the batter. Fill it evenly with the remaining jam, leaving a small border of just batter along the edges. Press remaining berries into jam, then spoon remaining batter on top and smooth out with the back of a spoon.
Bake until golden brown on top and edges start to pull away from pan, 65-75 minutes, or until a skewer comes out clean. Cool in pan for 10-15 minutes, but not much longer. If any berries did leak out, they will stick the cake to the pan as it cools and be difficult to remove in one piece.
Carefully invert cake onto a wire rack and cool completely. To finish, drizzle with lemon glaze and top with fresh berries.
Combine powdered sugar, sour cream, lemon juice and 1 tablespoon of milk in a medium bowl. Whisk until smooth. Add more milk, if needed, to reach a thick but pourable consistency.