Preheat oven to 350 degrees. Grease an 8-inch square baking pan.

In a mixing bowl, cream the butter and sugar at high speed until pale and fluffy. Add the eggs one at a time, beating thoroughly after each addition. Add the sour cream, beating until smooth. Sift together the flour, salt, baking powder, and baking soda and add to the egg mixture. Add the vanilla and blend thoroughly. Fold in the blackberries.

In a small bowl, combine the ingredients for the topping. Pour half of the batter into the prepared pan and sprinkle with half of the topping mixture. Add the remaining batter and sprinkle with the reaminder to the topping mixture. Bake approximately 35 minutes, or until a toothpick inserted in the center comes out clean.