Preheat oven to 350 degrees F. Butter and flour a 8 1/2 x 4 1/2 pan.
Mix together flour, baking powder and salt in medium bowl.
Add 1 tbsp of flour to the blueberries, toss to coat.
Using a stand mixer, whip together the butter, sugar and lemon zest until pale and fluffy.
Mix in eggs one at a time and then add the vanilla.
Combine the dry and wet ingredients alternately one third at a time.
Pour batter into the prepared pan, spread evenly and bake for 50 to 55 minutes.
Let cool and invert onto serving dish.