Preheat oven to 300 degrees. Lightly oil or spray a 8x8 baking pan with all natural cooking spray, set aside.
In a small bowl make flax eggs by mixing together flax meal and water. Let sit for 10 minutes or until a gel is formed.
In a large bowl add flax eggs, zucchini, sugar, milk, apple sauce, olive oil and vanilla, stir until well combined. In a separate bowl sift together gluten free flour, oat flour, baking powder, baking soda, cinnamon and sea salt. Combine wet and dry ingredients and fold in blueberries. Pour into 8x8 pan.
Meanwhile, make crumbled topping by adding oats, oat flour, brown sugar and salt to a bowl. Cut in butter until mixture is crumbly. Evenly spread on top of cake.
Place in oven and bake for 45-60 minutes or until knife inserted comes out clean.