Preheat oven to 350 F. Grease a 9x13" glass baking dish with non-stick cooking spray and set aside.
In a small bowl, combine all of the ingredients for the topping and mix well. Set aside for now.
Cake: combine the buttermilk, sugar, brown sugar, eggs, and melted butter. Mix well, and then add dry ingredients and lemon zest. Mix until everything is well-incorporated. Gently fold in the blueberries and stir.
Pour the cake batter into greased pan. Sprinkle the topping evenly over the cake batter.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean. If the top is starting to brown toward the end but the center is not cooked, loosely cover the cake with aluminum foil and continue baking until center is cooked through.
Remove cake from the oven and allow to cool completely. Once cooled, make the glaze: whisk together all the ingredients until smooth and creamy. Drizzle the glaze over top of the cooled cake.