Preheat the oven to 375°F and spray a 9x13-inch baking dish with nonstick spray.
In a large bowl combine the flour, baking powder, and salt.
In the bowl of a stand mixer with the paddle attachment, cream butter and sugar until fluffy (2-3 minutes). Add eggs, milk, vanilla, and oil; mix on low for about 1 minute, or until combined.
Add dry ingredients to the butter mixture and blend on medium for 30 seconds, or until combined.
Remove bowl from mixer. Gently fold the blueberries into the batter.
Pour batter into the prepared baking dish. Bake 30-35 minutes, or until browned and a toothpick comes out clean. (Cover the top of the cake with aluminum foil in the last 10 minutes if browning too fast.)
Set aside to cool slightly as you prepare the glaze.
In a medium mixing bowl, combine confectioners' sugar and milk. Whisk until light and smooth. Add more milk, if needed, to reach your desired consistency.
Drizzle glaze over the warm cake. Serve warm or at room temp.