Preheat the oven to 350°F (175°C). In a mixing bowl, combine blackberries, granulated sugar, cornstarch, and lemon juice. Toss gently to coat the berries evenly.

Transfer the berry mixture to a greased baking dish.

In a separate bowl, mix rolled oats, flour, brown sugar, sliced almonds, melted butter, and cinnamon until crumbly.

Sprinkle the topping evenly over the blackberry mixture in the baking dish.

Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and the filling is bubbly.

Remove from the oven and let cool slightly before serving.