Cream the butter and sugar with electric mixer until creamy & light in color.
Add the eggs (one at a time).
Add vanilla, lemon zest, sour cream, and milk.
Mix until well blended.
Using a sifter, sift together flour, baking soda, baking powder, and salt.
With the mixer on low, slowly add in dry ingredients until combined.
Add in cornmeal.
Lightly toss blueberries with 1TBSP of flour, then, using a spatula, carefully fold blueberries into batter (you don't want to crush the berries).
Spoon batter into prepared pan.
Top with streusel topping.
Bake for 40 to 50 minutes, until a cake tester comes out clean.
Cool completely and serve sprinkled with powdered sugar.