Preheat the oven to 350 °. Line 12-cup muffin pan with paper liners.
In a large mixing bowl combine the buttermilk, oil, and eggs. Whisk until well blended.
In another mixing bowl, combine the muffin mix and nutmeg. Whisk to blend. Add most of the blueberries, reserving a small handful for topping the muffins before baking.
Add the wet ingredients to the mixing bowl. Fold just until combined. It's okay if there are some dry streaks.
Divide the batter among 12 cups. It will heap up high. That's okay if it overflows. Top each muffin with a few of the reserved blueberries. Sprinkle with demerara sugar if you like.
Bake for 25 to 30 minutes or until a tester comes out clean. Cool the baked muffins on a wire rack.
Notes
To freeze muffins unbaked: Assemble the recipe as directed. Instead of baking, slide the pan into the freezer for several hours until the muffins are frozen solid. Remove the frozen muffin "pucks" from the pan and store them in an airtight container in the freezer. To bake, replace the frozen muffins into a muffin tin and bake, adding five minutes to the baking time if needed.To freeze baked muffins: Bake the muffins according to the recipe and cool completely on the rack. Place in an airtight container or ziptop bag and store in the freezer, for up to 2 months.Promptly store leftovers in an airtight container, for up to 3 days.Nutritional values are approximate and based on 1 muffin out of a 12-muffin batch.