Stir the blackberries and lemon juice together in a mixing bowl. In a separate bowl, whisk together all the other filling ingredients (sprinkle the glucomannan into the water while whisking to prevent clumping) and pour over the berries. Stir to combine. Pour the blackberry mixture into a greased 9x9 inch glass baking dish.
In a bowl, combine the topping ingredients, except the old-fashioned oats and stir until everything is evenly mixed. Spread over the top of the berry layer (your fingers will work the best for this). Sprinkle the old fashioned oats on top.
Bake at 350 degrees F for 40 minutes or until the topping is baked all the way through and not mushy in the center.
It's a good idea to let this cobbler sit for a little bit before serving to let the glucomannan thicken up.
Notes
This can be made with peaches or blueberries as well!