1To make topping:
In a medium bowl, combine brown sugar and cinnamon. Add walnuts and stir to combine. Add butter and mix with your fingers until mixture is moist and crumbly. Reserve.
2To make cake:
Preheat oven to 350 degrees. Grease bottom and sides of a 9-inch springform pan.
3In a large bowl, sift together flour, baking powder, soda and salt. In a large bowl using an electric mixer on medium high speed, cream butter and sugar until light and fluffy, about 2 minutes. Reduce speed to low and add eggs, one at a time, beating well after each addition.
4Add half of the sour cream and beat until combined. Add half of flour mixture and beat until combined.
5Repeat with remaining sour cream and vanilla, then add remaining flour and mix thoroughly.
6Using a spatula, gently fold in blueberries, stirring sparingly to keep batter from turning blue. Spread evenly in prepared pan.
7Distribute topping evenly over batter. Place springform pan on a baking sheet and bake about 55 to 60 minutes, until a toothpick comes out clean.
8Remove to a wire rack and cool. Release sides of pan, cut cake into wedges and serve warm or at room temperature.