Preheat the oven to 375 degrees F. Grease a 9x13-inch baking pan.
In a medium bowl, combine 4 cups of fresh blueberries and 3 tablespoons dark rum. If you prefer not to use rum, substitute an equal amount of fresh lemon juice instead.
Stir in ½ cup of granulated sugar and 4 teaspoons of cornstarch. Set aside.
In a separate medium bowl, combine ½ cup each of granulated sugar and brown sugar.
Stir in 3 cups of flour...
1 teaspoon baking powder, ¼ teaspoon salt...
½ teaspoon ground ginger...
...and ½ teaspoon ground nutmeg. If you can find yourself one these nifty little tools, they make grinding fresh nutmeg easier than pie.
Cut in 1 cup (8 ounces) of cold butter with a pastry cutter or a fork until the mixture is crumbly.
Whisk one egg in a small bowl and stir into the flour mixture.
Press half of the flour mixture into the bottom of the prepared pan.
Spread the blueberry mixture over the crust.
Sprinkle the rest of the flour mixture over the blueberries.
Bake in the preheated oven for 40-45 minutes, or until the top is light brown.
Left the bars cool on a rack and then place the fridge to chill. This will make the bars easier to cut and remove from the pan.