Heat oven to 350°F. Grease and line an 8-inch square pan with parchment.
Toss blueberries with 1 tbsp flour in a small bowl; set aside.
Whisk 1.9 cups flour, baking powder, and salt in a medium bowl.
Beat butter, sugar, and lemon zest until pale and fluffy, about 2 minutes.
Blend in eggs one at a time. Mix in vanilla, sour cream, and lemon juice.
Stir in dry ingredients alternately with buttermilk, beginning and ending with dry mix.
Fold in floured blueberries gently to avoid streaking.
Spread batter in pan. Bake until golden and a tester comes out clean, about 33–38 minutes.
Cool in pan 15 minutes. Lift out to a rack to finish cooling.
Whisk confectioners' sugar with lemon juice until smooth. Drizzle over cooled cake, then slice.