Preheat oven to 350 degrees F. Grease 13" by 9" metal baking pan; dust pan with flour.
Prepare Streusel Topping: In medium bowl, combine brown sugar, flour, and butter. With fingertips, work in butter until mixture is crumbly. Refrigerate streusel while preparing cake.
Prepare Coffee Cake: On waxed paper, combine flour, baking powder, baking soda, and salt. In large bowl, with mixer at low speed, beat sugar with butter until blended. Increase speed to medium-high; beat until creamy, about 2 minutes, frequently scraping bowl with rubber spatula. Reduce speed to low; add eggs, 1 at a time, beating well after each addition. Beat in vanilla.
At low speed, beat in flour mixture alternately with buttermilk, beginning and ending with flour mixture. Beat just until batter is smooth, occasionally scraping bowl with rubber spatula. With spatula, fold in all but 1/2 cup blueberries.
Spoon batter into prepared pan; spread evenly. Sprinkle batter with reserved blueberries. With fingers, press streusel into clumps; sprinkle evenly over batter. Bake coffee cake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool cake in pan on wire rack 1 hour to serve warm, or cool completely to serve later.