Preheat oven to exactly 350°F - use an oven thermometer if you don't trust your dial. Grease 9x13 inch pan thoroughly with butter, then dust with flour. Let eggs and milk come to room temperature while you prep. This prevents curdling and ensures even mixing in your lemon blueberry cake recipe.

Cream butter and sugar for a full 5 minutes until genuinely fluffy. Add eggs one at a time, beating well after each addition. Mix dry ingredients separately - flour, baking powder, salt. Alternate adding flour mixture and milk, starting and ending with flour. Don't overmix or you'll develop tough gluten in your moist lemon blueberry cake.

Zest lemons directly into the batter for maximum oil release. Add fresh lemon juice and sour cream for proper acid balance. Toss blueberries in flour until completely coated - this is critical. Fold berries in gently at the very end with minimal mixing. Coated berries won't sink and create purple disasters.

Pour into prepared pan and level the top gently. Bake 45-50 minutes without opening the door to peek. Test doneness with toothpick - should have just a few moist crumbs. Let cool in pan 10 minutes before turning out. Resist slicing until completely cool or it'll fall apart.