Add the blackberries, arrowroot starch, ground cinnamon, vanilla extract, lemon juice, lemon zest, and maple syrup to a mixing bowl. Stir until well combined and then transfer to the prepared baking dish.
Next, make the crisp topping. Add the oats, almond flour, brown sugar, ground cinnamon, vanilla extract and butter to a mixing bowl (you can use the same bowl you used to mix the filling). Use your hands to work the butter into the dry ingredients until you have a clumpy topping. Sprinkle the topping evenly over the fruit.
Bake the crisp at 350 degrees for 20-25 minutes (mine went for 25 minutes) until the top is golden brown and the fruit is bubbling around the edges.