Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan.

Sift flour, baking powder, baking soda, and salt together in a bowl. Set aside.

In a large bowl, cream together sugar and butter until light and fluffy. Beat in eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated. Sir in blueberries. Pour batter into prepared pan.

In a small bowl, combine flour and sugar. Cut in butter until mixture resembles coarse crumbs. Sprinkle over top of batter.

Bake in the preheated oven for 45 to 60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.