1. Preheat oven to 350°F. Grease a 12-cup Bundt pan with melted butter, add a few tablespoons of flour and rotate pan to evenly coat, tapping out excess.
2. Sift 3 cups flour with baking powder and salt into a large bowl. In a small bowl, combine lemon juice, vanilla and buttermilk.
3. In a second large bowl, beat butter, sugar and lemon zest until light and fluffy. Add eggs, one at a time, beating well after each. Add about 1/4 of flour mixture, followed by 1/4 of buttermilk mixture, mixing just until incorporated. Repeat, alternating dry and liquid ingredients, until batter is thoroughly combined. Gently fold in blueberries. Transfer batter to pan, spreading evenly.
4. Bake until a tester inserted into center of cake comes out clean, about 1 hour. Let cool in pan on a wire rack for 10 minutes, then unmold cake.
5. Make glaze: Mix confectioners' sugar, buttermilk and 2 Tbsp. lemon juice in a bowl, stirring until smooth. Glaze should be pourable; if it's too thick, add lemon juice, 1 tsp. at a time, until it reaches desired consistency. Pour half of glaze over warm cake. Let cool for 1 hour, then pour remaining glaze over cake and serve.