Preheat an oven to 180 degrees C. Grease a 23x23cm baking tin.
Whisk 1/2 cup sugar together with the flour and baking powder in a mixing bowl. Stir in the milk and margarine until a batter forms. Scrape into the prepared baking tin. Add the blackberries and sprinkle with 1/2 cup sugar. Pour in the water.
Bake in the preheated oven until the batter has risen and the top is brown, 45 minutes to 1 hour. Cool before serving.