Preheat oven to 350 degrees. Cut the English muffins in 1-inch cubes and place on a cookie sheet. Lightly toast in the oven for 5-8 minutes, turning once in the oven. Spray a 9x13 pan with cooking spray. In a large mixing bowl, combine milk, egg yolks, whole eggs, and vanilla. Mix ingredients well. Fold in the blueberries. Add cubed, toasted muffin pieces to soak up the mixture. Pour melted butter into the 9x13 pan, coating the bottom entirely. Spoon the egg mixture into the pan. Sprinkle with the dark brown sugar. Bake for 55 minutes, or until French toast has set and browned. Served warm with 100% maple syrup. Optional to dust with powdered sugar.