1. To prepare crumb topping, place flour, sugar, pecans and salt in bowl of a food processor. Pulse to blend. Add butter; pulse until large, moist crumbs begin to form. Set topping aside. Cover and chill.
2. Preheat oven to 350°. To prepare cake, combine flour, cornmeal, baking powder, baking soda, and salt in a medium bowl; set aside.
3. Using an electric mixer on medium speed, beat ¾ cup sugar and butter in a medium bowl until light and fluffy, 3–4 minutes. Beat in vanilla. Add eggs one at a time, beating well after each addition.
4.With mixer on low speed, add dry ingredients to bowl alternating with buttermilk beginning and ending with dry ingredients. Pour half of batter into greased 9-inch pan and smooth top.
5. Whisk remaining 3 tablespoons sugar and cinnamon in a small bowl; sprinkle evenly over batter in pan. Spoon remaining batter over; smooth top.
6. Scatter blueberries evenly over batter. Sprinkle reserved crumb topping over blueberries. Bake cake until top is golden brown and a tester inserted into the center comes out clean, 55–65 minutes. Let cool completely in pan.