Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line the bottom with parchment paper.
In a medium bowl, whisk together the gluten free flour, baking powder, baking soda, and salt.
In a large bowl, beat the softened butter and granulated sugar for 2 to 3 minutes, until light and creamy.
Beat in the eggs one at a time, then mix in the vanilla extract.
Add the dry ingredients in portions, alternating with the sour cream and milk. Mix just until the batter comes together. Do not overmix.
Toss the blueberries gently with 1 tablespoon of gluten free flour. Fold them carefully into the batter with a rubber spatula.
Spread the batter evenly into the prepared baking pan.
For the crumb topping, mix the gluten free flour, brown sugar, cinnamon, and salt in a small bowl. Stir in the melted butter until coarse crumbs form.
Sprinkle the crumb topping evenly over the cake batter.
Bake for 40 to 48 minutes, or until the top is golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan for 15 to 20 minutes before transferring it to a wire rack.
If using the glaze, whisk the powdered sugar, milk, and vanilla until smooth. Drizzle over the cooled cake before serving.
