Preheat your oven to 350°F (175°C). Grease a 9×5-inch loaf pan with butter or line it with parchment paper.
In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, baking soda, salt, and lemon zest until well combined.
In a separate bowl, whisk together the melted butter, lemon juice, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing well until a smooth batter forms.
Gently fold in the blueberries into the batter, being careful not to overmix.
Pour the batter into the prepared loaf pan, spreading it evenly.
Bake the bread in the preheated oven for 45-50 minutes or until a toothpick inserted into the center comes out clean