Preheat your oven to 375°F (190°C). In a mixing bowl, gently toss the blackberries with granulated sugar and lemon juice until evenly coated. Transfer the berry mixture to a greased baking dish, spreading it out evenly.
In a separate bowl, combine the oats, flour, and brown sugar. Mix well to distribute the ingredients evenly.
Add the cold cubes of butter to the dry mixture. Use a pastry cutter or forks to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces.
Sprinkle the crumble mixture evenly over the berries in the baking dish, covering the fruit completely.
Place the baking dish in the preheated oven and bake for about 25 minutes, or until the topping is golden brown and the berries are bubbling around the edges.
Remove from the oven and let cool slightly for about 10 minutes. The filling will thicken and become jammy as it cools, and the topping will stay crisp and fragrant.