Preheat the oven to 375 degrees F (190 degrees C). Grease an 8x8-inch pan.
Beat sugar, butter, and egg in a large bowl with an electric mixer until light and fluffy. Stir flour, baking powder, and salt together in a separate bowl. Add flour mixture to sugar mixture in 2 batches, alternating with milk, beating batter briefly after each addition. Fold in blueberries. Pour into the prepared pan.
To make the topping: Combine sugar, flour, butter, and cinnamon in a bowl; mix with your hands until crumbly. Sprinkle over cake batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 40 minutes.
