Preheat your oven to 180C/350F and line a muffin pan with paper liners. Add the room temperature butter and cream cheese to a bowl. Beat using a hand blender until incorporated.
Add the granulated erythritol and lemon zest and beat until creamy. Add the eggs one at a time and beat until incorporated. Add the vanilla and lemon juice and beat again.
In another bowl, mix the almond flour, coconut flour, baking powder, and salt. Add the dry mixture to the wet, and fold to form a batter. Roughly chop ½ cup of blackberries and fold them in.
Divide the batter into the prepared muffin pan. Add the remaining ¼ cup of berries to the top of the cupcakes and press them in slightly. Sprinkle the top with some granulated erythritol.
Transfer the pan to the oven and bake for 20-25 minutes until lightly browned and a toothpick inserted comes out clean. Transfer the muffins to a cooling rack and let cool completely. Serve warm or at room temperature.