Position a rack in the center of the oven and preheat to 350°F. Generously spray an 11x7-inch ovenproof glass baking pan with oil.

In a medium bowl, toss together the blackberries, sugar, cornstarch, and lemon juice. Transfer to the prepared baking dish. Set aside.

In another medium bowl, whisk the flour, baking powder, baking soda, and salt until well combined. Set aside.

In a third medium bowl, using a handheld electric mixer set at high speed, beat the buttermilk, sugar, egg, oil, and vanilla until frothy, about 2 minutes. Make a well in the center of the dry ingredients and pour in the buttermilk mixture. Stir with a wooden spoon just until combined. Do not overmix.

Drop large tablespoonfuls of the batter over the berries. Bake until the topping is golden brown, about 30 minutes. Do not overbake. Cool slightly in the pan on a wire cake rack, then serve warm.