Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Sift together 4 cups flour, baking powder, baking soda and salt. Set aside.
In a large bowl, cream together 1/2 cup butter and 1 1/3 cup sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated. Stir in blueberries.
Pour batter into prepared pan. In a small bowl, combine 2/3 cup flour and 2/3 cup sugar. Cut in 6 tablespoons 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.