Step 1: Preheat oven to 375°F (190°C). Lightly grease an 8x8 inch (or similar 2-quart) baking dish.
Step 2: In a large bowl, gently combine the blackberries, granulated sugar, 2 tablespoons all-purpose flour, and lemon juice. Toss until the berries are evenly coated. Pour the berry mixture into the prepared baking dish.
Step 3: In a separate medium bowl, whisk together 1 cup all-purpose flour, rolled oats, brown sugar, cinnamon, and salt for the crisp topping.
Step 4: Add the cold, cubed butter to the dry topping ingredients. Using a pastry blender, your fingertips, or a fork, cut the butter into the flour mixture until coarse crumbs form. A few larger pea-sized pieces are fine; they will melt and create pockets of deliciousness in your blackberry crisp recipe.
Step 5: Evenly sprinkle the oat topping over the blackberry filling in the baking dish.
Step 6: Bake for 35-45 minutes, or until the topping is golden brown and the berry filling is bubbly around the edges. If the topping starts to brown too quickly, you can loosely tent it with aluminum foil.
Step 7: Remove the blackberry crisp from the oven and let it cool for at least 15-20 minutes before serving. This allows the filling to set slightly and prevents it from being too runny.