In a large mixing bowl, beat together the milk, sugar, eggs, vanilla, lemon juice & zest, salt, and flour.
Pour a ¼ inch layer of the batter in a round baking dish or deep dish pie pan coated with cooking spray. Place in the oven until the batter firms up a bit, about 10-12 minutes.
Remove from the heat and place the blackberries evenly over the batter. Pour the rest of the batter evenly over top. Bake for another 20 minutes, and then top with the candied lemon slices if you are adding them.
Bake for another 30-40 minutes. The clafoutis is done when it is puffed and golden brown and a toothpick inserted in the center comes out clean.
Serve warm or at room temperature. Feel free to dust with powdered sugar or top with whipped cream. Cover tightly & refrigerate any leftovers.