1For the Streusel: Using stand mixer fitted with paddle, combine flour, brown sugar, granulated sugar, cinnamon, and salt on low speed until well combined and no large brown sugar clumps remain, about 45 seconds. Add butter; beat on low speed until mixture resembles wet sand and no large butter pieces remain, about 2½ minutes. Transfer to bowl; set aside.
For the Cake: Preheat oven to 350°F and grease and flour a 9″ round cake pan.
Whisk flour and baking powder together in bowl; set aside. Using stand mixer fitted with paddle, beat butter, sugar, salt, and zest on med-high speed until light and fluffy, about 3 minutes, scraping down bowl as necessary. Beat in vanilla until combined, about 30 seconds. With mixer on medium speed, add eggs 1 at a time; beat until partially incorporated, scrape down bowl, and continue to beat until fully incorporated (mixture will appear broken). With mixer on low speed, gradually add flour mixture; beat until flour is almost fully incorporated, about 20 seconds. Stir batter with rubber spatula, scraping bowl, until no flour pockets remain and batter is homogeneous; batter will be very heavy and thick. Gently fold in blueberries until evenly distributed.
Transfer batter to pan. Spread batter evenly to pan edges and smooth surface. Squeeze portion of streusel in hand to form large cohesive clump; break up clump with fingers and sprinkle streusel evenly over batter. Repeat with remaining streusel.
Bake until cake is deep golden brown and toothpick comes out clean, about 55 minutes. Transfer to wire rack and let cool.