Preheat the oven to 450°. Spread 1/2 cup of the blackberries in each of twelve 4-ounce gratin dishes. Arrange the gratin dishes on 2 baking sheets.
In a bowl, whisk the milk with the cream, eggs, brown sugar, flour, vanilla, lemon juice and orange zest until smooth. Ladle 1/4 cup of the custard into each dish; bake for about 15 minutes, until the centers are set. Transfer the custards to wire racks and let stand for 30 minutes. Serve.