Preheat oven to 350.
Line an 8x8 baking dish with parchment paper (grease inside of pan first so the parchment paper sticks).
Make the topping first, by combining all ingredients in a medium bowl.
Mix well and set aside.
Now make the cake by adding all of the ingredients, except the blackberries, and blend until well incorporated.
Transfer to a medium bowl, and fold in the blackberries.
Mix well, and you can go ahead and crush some of the blackberries in the batter.
Pour half of the batter into the baking dish, followed by half of the topping.
Then add remaining batter and remaining topping.
Use a spatula to flatten and make even.
Bake for 40 minutes.