Stir together the dry cake ingredients in a medium bowl. Add all the other cake ingredients and mix well with a wooden spoon. Spread in a greased 8 x 8-inch baking dish. Sprinkle the blackberries on top of the cake batter.
Bake for 15 minutes.
Meanwhile, combine the ingredients for the topping in a food processor. Process until the nuts are chopped and wet crumbs form. Sprinkle evenly over the partially baked cake.
Bake for an additional 40-55 minutes until the cake is firm to the touch, golden, and doesn't jiggle when lightly shaken. The center should feel as firm as the edge when lightly pressed with your fingertip.