This low-carb blueberry crumb cake recipe is perfect for those who are watching their carb intake. The cake is made with almond flour and coconut flour, which are low in carbs and high in fiber. The blueberries add a burst of sweetness and color to the cake. The crumb topping is made with almond flour and erythritol, a natural sweetener that doesn't raise blood sugar levels. Serve it with a cup of unsweetened almond milk or black coffee for a perfect low-carb breakfast or snack.