Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another large bowl, cream together the softened butter and sugar until light and fluffy. Beat in eggs one at a time.
Gradually add dry ingredients to wet ingredients, alternating with milk, mixing until just combined.
Stir in vanilla extract. Gently fold in fresh blueberries.
Pour batter into prepared pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let cake cool in the pan for 10-15 minutes before inverting onto a wire rack to cool completely.
For frosting, cream together softened butter and powdered sugar until smooth. Gradually add milk and vanilla extract, beat until fluffy.
Once cake is completely cool, frost and decorate as desired.