Grease and flour a loaf pan. Preheat the oven to 350°F.
Wash the lemons with hot water and zest one of them. Then juice both lemons, set the juice aside.
Cream together the butter and 1¾ cups sugar. Beat in the eggs one at a time.
Add the flour, baking powder, lemon zest, vanilla sugar, ¼ teaspoon of salt, and milk.
Stir in 3 tablespoons of lemon juice and gently fold in ⅔ of the blueberries.
Pour the batter into the loaf pan and sprinkle with the remaining blueberries.
Bake for an hour. Cover with aluminum foil about 15 minutes before the end of the baking time.
Boil the remaining lemon juice with ⅓ cup sugar to make syrup.
Remove the bread from the oven, prick it several times with a wooden skewer, and drizzle with the syrup.
Let the bread cool in the pan for 30 minutes, then invert it before serving.
