1. In a bowl, gently mix blackberries, 1/2 cup raspberries, port, and 1 tablespoon sugar.
2. Butter and flour a 9-inch cake pan with removable rim.
3. In a bowl with a mixer on high speed, beat 1 cup sugar and 1/2 cup butter until well blended, 2 to 3 minutes. Add eggs and beat until fluffy, 2 to 3 minutes.
4. Add 1 cup flour and baking powder. Stir to combine, then beat on high speed until the stiff batter is well blended, about 2 minutes.
5. Scrape batter into cake pan and spread top smooth. Drain wine marinade from berries and save. Evenly spoon berries and 2 tablespoons of the marinade over batter.
6. Bake in a 350° oven until cake begins to pull from pan rim, 50 to 55 minutes (40 to 45 minutes in a convection oven). Run a thin-bladed knife between cake and pan rim. Let cool at least 10 minutes.
7. Remove pan rim and sprinkle cake with a little more sugar. Top with remaining raspberries, cut into wedges, and moisten portions with reserved wine marinade. Accompany with scoops of ice cream.