Spray the inside of your slow cooker with non-stick cooking spray.
In a bowl, toss blueberries with 1/4 cup erythritol, lemon juice, and lemon zest, then spread evenly in the bottom of the slow cooker.
In a large mixing bowl, combine almond flour, coconut flour, remaining 1/4 cup erythritol, baking powder, and salt.
In another bowl, whisk together melted butter, eggs, almond milk, and vanilla extract.
Pour wet ingredients into dry ingredients and stir until just combined.
Drop spoonfuls of the batter over the blueberries, spreading gently to cover most of the fruit.
Cover and cook on high for 3 hours, or until the topping is set and the blueberries are bubbling around the edges.
Turn off the slow cooker, remove the lid, and let cool for 15 minutes before serving.