How to Make It
Preheat the oven to 350°. Spray a 10-inch cake pan with cooking spray and line the bottom with parchment paper. In a bowl, whisk the flour, sugar, salt, baking soda and powder. In another bowl, whisk the olive oil, milk, eggs, orange zest and Grand Marnier. Add the dry ingredients; whisk until just combined.
Pour the batter into the prepared pan and bake for 1 hour, until the top is golden and a cake tester comes out clean. Transfer the cake to a rack and let cool for 30 minutes. Run a knife around the edge of the pan, invert the cake onto the rack and let cool completely, 2 hours.
In a bowl, toss the blackberries, sugar, oil and zest. Let stand, tossing occasionally, until the sugar dissolves and the berries release some of their juices, about 30 minutes.
Cut the cake into wedges. Transfer to plates, spoon the blackberries alongside and serve.