Preheat oven to 350°F. Spray two 8.5 x 4.5-inch loaf pans with cooking spray and set aside.
In a large bowl, use an electric mixer to cream together the butter and sugar. Add the eggs, one at a time, and mix until incorporated. Slowly add the milk and the lemon flavor (if using); mix to combine. In a separate bowl, sift together the flour, baking powder and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to over-mix the batter.
Gently fold in the blueberries.
Divide batter evenly between the two loaf pans. In a small bowl, combine the sugar and cinnamon for the topping. Sprinkle over the top of the bread.
Bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean. If the top starts to get too dark, after the first 45 minutes you can loosely cover the bread with foil until it's done baking. Remove from the pans and cool completely on a cooling rack before slicing and serving.
Notes
Aunt Mary always uses 1-2 teaspoons of lemon extract. If you prefer, you can substitute with 2 tablespoons of grated lemon zest and 2 tablespoons of freshly-squeezed lemon juice. If you don't want lemon blueberry bread, just omit this ingredient altogether. The bread is delicious even without the citrus flavor.
Bake a blueberry cake in a Bundt pan (instead of using the two loaf pans). Bake the Bundt cake at 325 degrees F for 70-80 minutes. Cool the cake in the pan for about 20 minutes, and then transfer to a wire rack to cool completely.
Instead of the cinnamon-sugar topping, try a lemon glaze. Simply whisk together 1 ¼ cups confectioners sugar with 2-3 tablespoons of lemon juice and 1 teaspoon of vanilla extract. Drizzle the glaze over the cooled loaves.
If using unsalted butter, add an extra ¼ teaspoon of salt to the batter.