Heat the oven to 350 °F and thoroughly grease a mini muffin pan. The extra greasing helps the rounded edges release cleanly after baking.
Whisk the flour, rolled oats, baking powder, baking soda, salt, and sugar together in a medium bowl. An even dry mix helps the batter rise uniformly instead of puffing unevenly.
Stir the buttermilk, egg, maple syrup, and melted butter in a second bowl until combined. The mixture should look smooth and lightly creamy, not foamy.
Add the wet mixture to the dry ingredients and stir with a spoon just until the batter comes together, then fold in most of the blueberries and save a few for the tops. Stop mixing as soon as no large dry patches remain so the poppers stay light instead of dense.
Spoon the batter into the mini muffin pan and press the reserved blueberries onto the tops. Do not overfill the cups or the batter can spill over and lose the rounded popper shape.
Bake for 10 to 12 minutes. The batter should puff up and turn light golden brown around the edges.
Open the oven carefully, watch for hot steam, and test a center popper with a toothpick before removing the pan. If the toothpick comes out clean, the inside is baked through.
Let the poppers cool briefly, then run a toothpick around the edges and lift them out. They should release with a gentle nudge once the sides pull away from the pan.