Preheat the oven to 350 degrees Fahrenheit and spray a 3-quart baking dish with pan spray.
In a medium bowl, gently toss the blackberries with sugar, flour, cinnamon, salt, and lemon juice until well combined. Transfer the mixture into the prepared baking dish.
In another bowl, prepare the crumble topping by whisking together flour, sugar, cinnamon, and salt. Cut in the softened butter using a pastry cutter until the mixture resembles coarse breadcrumbs.
Sprinkle the crumble topping evenly over the blackberry mixture in the baking dish.
Bake for 25-30 minutes, or until the top is golden brown and the filling is bubbling around the edges.
Allow the blackberry crumble to cool for 5-10 minutes before serving. This will allow the filling to cool slightly and thicken.
Serve warm, and optionally, top with a scoop of vanilla ice cream, whipped cream, or crème anglaise for an extra indulgent treat.