In a small bowl, sift together the almond flour, coconut flour, baking soda, and xanthan gum. In another small bowl, mix together the butter, egg, and brown sugar until thoroughly combined.
Pour the wet ingredients into the dry ingredients and mix until homogenous. Fold in the blueberries, lemon zest, and lemon juice, then transfer the batter to a 10-to 12- ounce microwave-safe serving dish. Cover with plastic wrap and microwave on high for 1 minute.
Remove your keto blueberry mug cake from the microwave.
Option to garnish the keto blueberry mug cake with powdered erythritol. Serve immediately or let cool, cover, and refrigerate for up to 1 week.