Prep the Berries. In a medium bowl, toss the blackberries with sugar, cornstarch, lemon juice, vanilla, and a pinch of salt. Stir gently until combined, then pour into a greased 8x8-inch baking dish.
Make the Crisp Topping. In another bowl, mix oats, flour, brown sugar, cinnamon, and salt. Add the cold cubed butter and use a pastry cutter or your fingers to cut the butter into the mixture until it resembles coarse crumbs.
Assemble and Bake. Sprinkle the crisp topping evenly over the blackberries. Bake in a preheated oven at 375°F for 35–40 minutes, or until the top is golden and the berries are bubbling around the edges.
Cool and Serve. Let the crisp cool for at least 10 minutes before serving to allow the juices to thicken. Top with ice cream or whipped cream, if desired.