Place the almond flour, coconut flour, sweetener, baking powder, and salt in a medium-large microwave-safe mug and blend with a fork.
Add the egg, butter, avocado oil, and vanilla; mix well.
Gently stir in the blueberries. Use the back of a spoon to pressthe batter down and smooth the top.
Place the batter-filled mug in the microwave and heat for 1 minute 15 seconds. (The cooking time may vary depending on your microwave. If the muffin is not fully formed after 1 minute 15 seconds, continue cooking in 15-second increments.) Carefully remove the mug from the microwave—it will be hot—flip it upside down over a plate, and allow the muffin to slide out of themug onto the plate. Place the muffin on its side and slice in half.
Spread with butter, if desired, and enjoy!