Preheat oven to 325 degrees Fahrenheit.
Mix all crust ingredients together.
Line a 9"x9" pan with parchment paper and press half the crust mixture into the bottom of the pan, making it smooth and flat.
Bake the crust for 12 minutes.
Meanwhile, mash the blackberries in a pot on medium heat with all of the filling ingredients. Stir regularly for 8-10 minutes and let it reduce down. When it looks like jam, it's READY!
Carefully spoon the blackberry mixture on top of the baked crust and then gently press down the remainder of the oat mix crust on top (it doesn't have to be perfect, just try to cover as much as possible).
Bake for 15 minutes. Cool FULLY, then lift the parchment paper sides and lift out onto a cutting board and cut into squares.
Store in an airtight container in the fridge for up to a week.