Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
Whisk together the flour, baking powder, and salt in a medium mixing bowl.
In a separate large mixing bowl, combine the softened butter and granulated sugar. Cream them together until the mixture becomes light and fluffy.
Add the eggs to the mixture one at a time, making sure to beat well after each addition. Stir in the vanilla extract.
Slowly incorporate the dry ingredients into the butter mixture, alternating with the milk. Begin and end with the dry ingredients, and mix until the ingredients are just combined.
Gently fold in the fresh blueberries.
Pour and spread the batter into the prepared baking dish evenly.
In a small bowl, combine the flour and granulated sugar for the streusel topping. Using a pastry cutter or your fingertips, cut the chilled butter pieces into the mixture until it resembles coarse crumbs.
Evenly sprinkle the streusel topping over the cake batter.
Place the baking pan in the preheated oven and bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Once baked, remove the coffee cake from the oven and let it cool in the pan for about 10 minutes before serving.