Place all of the cake ingredients (minus the blueberries) into a mixer, and mix on low speed for 2 minutes - or until well combined. Gently fold in the blueberries to the batter.
Mix together the streusel ingredients with a pastry blender or your fingers until the mixture is crumbly and combined. Sprinkle on top of the cake batter.
Prepare an 8x8 baking pan with baking spray, and spoon the batter into the prepared pan. Bake for 45-55 minutes, or until a tester comes out with moist crumbs.