Preheat oven to 350 degrees and prepare 2 (10-inch) loaf pans by buttering and flouring them well.
Cream butter, sugar and lemon zest until light and fluffy, about 4 to 5 minutes. Add eggs, one at a time, mixing well after each addition. Add milk and vanilla and mix until combined.
Sift flour and baking powder together and slowly add to wet ingredients. Beat just until flour is incorporated into batter. Gently fold in blueberries by hand.
Divide batter between the two loaf pans and bake for 55 to 60 minutes or until the top is golden brown and a toothpick comes out clean.
While loaves are baking, make lemon syrup by combining 1/3 cup lemon juice and sugar in a small saucepan and simmering over medium heat until slightly reduced and thickened, about 5 minutes. Set aside to cool.
Let loaves cool in the pans for 10 minutes. Remove to a wire rack and cool another 10 minutes. Using a long skewer, poke holes all over the loaf, being careful not to go all the way through.
Brush the cooled syrup over the loaves. Let loaves cool the rest of the way, at least an hour.
Whisk together sifted powdered sugar and 2 tbsp. lemon juice to form a glaze. Pour over cooled loaves and serve immediately.
Makes 2 loaves