This cake is packed full with juicy blueberries both inside and on-top!
For the cake: Adjust an oven rack to the middle position and heat the oven to 350°F. Grease and flour a 13- by 9-inch baking pan. Whisk 2 cups flour, baking powder and salt in a medium bowl. With an electric mixer on medium-high speed, beat the butter and sugars until fluffy, about 2 minutes. Add the eggs, one at a time, beat in until just incorporated. Reduce the speed to medium and beat in the flour mixture and the milk alternately in tow batches until incorporated. Toss the blueberries with the remaining 1 teaspoon flour. Using a rubber spatula, gently fold in the blueberries. Scrape the batter into the prepared pan.
For the topping: Scatter the blueberries over the top of the batter. Stir the sugar and cinnamon together in a small bowl; sprinkle over batter. Bake for 45 to 50 minutes, or until toothpick inserted in center comes out clean. Cool cake in pan for 20 minutes before taking out of pan