Preheat oven to 180 degrees C. Grease and flour a 23cm square pan.
In a large bowl combine flour, sugar, baking powder and salt. Cut in margarine until mixture resembles coarse crumbs. Set aside 3/4 cup of crumb mixture to be used as a topping for the cake. Mix eggs and milk together and then blend into remaining mixture in bowl.
Spread batter into prepared pan. Sprinkle blackberries evenly over the top. Sprinkle reserved crumb mixture over fruit.
Bake in the preheated oven for 25 to 30 minutes or until a toothpick inserted into the centre of the cake comes out clean.