Put the almond flour, coconut flour, and butter in a food processor and pulse until crumbs form. Add the water and pulse until it comes together in a dough. Press into the bottom and up the sides of a pie plate. Prick the bottom with a fork. Bake for 10 minutes.
Remove from the oven and scatter the blackberries over the crust.
Combine the rest of the ingredients in a blender and blend until smooth. Pour the custard over the berries.
Bake for 40-50 minutes until the center isn't jiggly and the pie is golden brown. Cool completely and then chill in the fridge for a few hours before serving.