Preheat oven to 375 degrees.
Grease an 8-inch square pan.
In a large bowl, cream the butter and ¾ cup sugar. Add the egg and mix until blended in.
In a separate bowl, mix together the 1½ cups flour, baking powder and salt.
Add the flour mixture alternately with milk to creamed mixture, beating well after each addition.
Gently fold in the blueberries.
Pour into the prepared pan.
Combine the ½ cup sugar, ⅓ cup flour, cinnamon and ¼ cup butter until crumbly. Sprinkle over the batter.
Bake for 40 to 45 minutes or until a toothpick inserted into the middle comes out clean. Cool on a rack for 15 minutes before cutting into pieces and serving.