Preheat oven to 425F / 220C Add all wet ingredients (including granulated allulose) and whisk well (use an electric mixer if you have one).
Sift together all dry ingredients (except blueberries) in a separate bowl, then add to wet ingredients and mix until combined. Cover and refrigerate for at least 30 minutes.
Prepare muffin pan with liners, then add ~20g of plain batter into each muffin.
Toss blueberries in a small amount of flour to coat, sift off excess, then add to remaining batter, fold them in, and scoop evenly into each muffin.
Prepare brown sugar topping by mixing all ingredients together with a fork (or pastry cutter), then sprinkle evenly over all muffins
Place in oven for 7 minutes at 425F / 220C, then reduce heat to 350F / 175C and bake for another 12 minutes.
Let cool in pan for 5 minutes, then remove and place on wire rack. Enjoy hot or let them cool completely.