Start by preparing the crumb topping so it can chill in the fridge while the batter is prepared.
In a medium bowl combine the flour, sugar, brown sugar, salt, and cinnamon. Dice the cold butter into small cubes and add to the flour and sugar mixture. Use a fork or pastry cutter to cut in the butter until crumbly. Set in the fridge until needed.
Preheat the oven to 375°. Spray an 8x8 baking pan with pan spray and set aside.
In a medium bowl whisk together the flour, baking powder, and salt.
In a large bowl beat together the sugar and butter until creamy. Add the egg and mix until well combined. Add in the flour mixture and milk alternating, starting with the flour. Once the flour and milk are fully incorporated into the batter, gently fold in the blueberries until just barely evenly distributed.
Pour the batter into the prepared pan and top evenly with the chilled crumb topping. Bake for 30-35 minutes, until a toothpick, comes out clean from the center. Let cool before serving.