Preheat oven to 350*F and line a 9"x 5" loaf pan with parchment paper (or lightly grease with butter).
In a medium bowl, whisk the flour, baking powder and salt, and set aside.
In the bowl of an electric mixer, blend together the melted butter, sugar, eggs, vanilla, lemon zest and lemon juice. Mix until well combined.
While slowly mixing, add flour mixture and milk in two batches (some flour, then some milk, then the rest of the flour and the rest of the milk). Stop mixing as soon as it's just combined.
Rinse off the blueberries (if using fresh) so they have just a bit of moisture on them, then, in a small bowl toss the blueberries and 1 T. flour. This flour coating will help prevent the blueberries from sinking to the bottom of your loaf while baking.
Add the flour coated berries to the batter and gently but quickly stir, by hand, to combine.
Immediately pour batter into prepared pan and bake for 55-65 minutes, or until a toothpick inserted in the center of the loaf comes out clean. Cool bread in the pan for about 30 minutes, then move to a wire cooling rack with a baking sheet below (to catch the glaze you're about to add).
Prepare glaze by simply whisking together the melted butter, powdered sugar, lemon juice and vanilla, then pour glaze over the loaf. Allow to set a few minutes, then enjoy!
If baking the day before serving, wrap cooled loaf in plastic wrap and store at room temperature WITHOUT the glaze. Then, mix up and add the glaze the day of serving.