Preheat oven to 375°, and generously grease a 9-inch square or round pan.
In a bowl, combine the flour, baking powder, and salt; stir with a fork to mix well.
In another bowl, combine the butter and sugar, beat with a mixer at high speed until well combined.
Add in egg and beat well for 1-2 minutes, stopping to scrape down the bowl, until the mixture is smooth and light.
Stir in half the flour mixture, then half the milk, mixing just enough to keep the batter fairly smooth and well combined.
Add the remaining flour, then the rest of the milk, mixing gently.
Stir in the blueberries.
Scrape the batter into the prepared pan, bake for 30 minutes or until the cake is golden, springs back when touched gently in the center and is pulling away from the sides of the pan.
Serve a square cake right from the pan, warm or at room temperature; if it is round, let the cake cool in the pan on a wire rack for 10 minutes; then turn it out to cool on a wire rack, top side up.