1. Preheat oven to 350°F/180°C.
2. Have ready a 9-inch round oven dish (ceramic or glass) or an 8x11 rectangular baking dish. 
3. If using store-bought blackberry pie filling, you only have to open the can and dump it in the dish, spreading it to cover the bottom. 

Filling from scratch:

1. Mix blackberries with sugar and lemon juice in the baking dish or in a separate medium bowl and then transfer to the dish. 
2. Stir the cornstarch with the water in a small bowl or cup until completely dissolved. 
3. Drizzle into the berry mixture and stir to integrate. 

For the topping:

1. Pour the melted butter over the cake mix in a medium bowl and stir until you have a crumbly mixture. It might be floury in parts, and that is fine. 
2. Sprinkle over the blackberries. It will not cover it completely, and that is fine. Don't stir it and mix it with the berries. 
3. Bake for 40-45 minutes, until golden and dry and the filling bubbles around the edges, checking close to the end of the baking time by lifting here and there and making sure the topping is fully baked. If necessary, bake it longer. 
4. Serve warm plain, with a scoop of ice cream or dollop of whipped cream. And fresh blackberries if you want to.