Preheat oven to 375 degrees. Grease and flour a 9" square pan. Prepare crumb topping by combining crumb ingredients in a bowl. With a pastry fork, cut in butter until crumbly. Set aside.
To prepare cake: cream the butter and sugar together until well blended. Beat in the egg and vanilla until batter is smooth. In separate bowl mix together flour, baking soda and cinnamon. Alternating with the buttermilk, add the flour mixture to the egg mixture, beating well after each addition. Begin and end with the flour. Carefully stir in blueberries. Spread batter into prepared pan. Sprinkle crumb topping over batter. Bake for 55 minutes or until toothpick comes out clean. Cool and cut into pieces.