Preheat the oven to 335°F (170°C). Coconut flour can start to burn at 350°F, so the slightly lower temperature prevents browning the tops too quickly.
A silicone muffin pan is ideal because it doesn't need cupcake liners. If using a metal pan, line with cupcake liners or grease the wells well.
In a large bowl, whisk together the coconut flour, oat flour, baking soda, baking powder, and sea salt.
In a medium bowl, crack the eggs and whisk vigorously for about a minute. Add the apple cider vinegar, coconut milk, avocado oil, honey, and vanilla extract. Whisk for another 30 seconds to fully combine.
Pour the wet ingredients into the dry ingredients and stir with a whisk until just combined. Do not over-mix — over-mixing leads to dense muffins.
Gently fold in the blueberries.
Using two spoons, divide the batter evenly into the muffin pan. You should get 7–8 muffins depending on how full you fill each cup.
Bake at 335°F for 25–30 minutes. Test with a toothpick — it should come out clean (or with a few moist crumbs) when the muffins are done.
Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool for at least 15 minutes more. Serve warm, or store in an airtight container for up to 3 days.