Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a large mixing bowl, cream together the softened butter and 3/4 cup of granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Mix until just combined.

Gently fold in the fresh blueberries.

In another bowl, combine the brown sugar, remaining 1/4 cup of granulated sugar, cinnamon, and rolled oats to make the crumb topping.

Pour the cake batter into the prepared pan and evenly sprinkle the crumb topping over the batter.

Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.