Start by preheating the oven to 375°F (190°C). Prepare a 9-cup muffin tray by lightly greasing it or lining it with muffin liners.
In a medium bowl, combine the almond milk and apple cider vinegar. Stir well and let it rest for a few minutes until it slightly thickens.
In a large mixing bowl, add oat flour, almond flour, protein powder, baking powder, baking soda, salt, and cinnamon. Mix until everything is evenly blended.
In the bowl with the almond milk mixture, add egg (or egg whites), Greek yogurt, coconut sugar, and vanilla extract. Whisk until smooth and well combined.
Pour the wet mixture into the dry ingredients. Stir gently until just combined. Avoid overmixing to keep the texture light and fluffy.
In a small bowl, lightly coat the blueberries with 1 tablespoon of oat flour. Then fold them gently into the batter so they spread evenly.
Divide the batter evenly into 9 muffin cups. Add a few extra blueberries on top if you want.
Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Let the muffins rest in the pan for about 5 minutes, then transfer them to a cooling rack.