Preheat the oven to 350 degrees F and line a 9x13 inch pan with parchment paper.
In a large bowl, combine the flour, sugar and salt. Stir in the melted butter until well combined. It will be rather crumbly.
Set aside 1 cup of the mixture for the topping. Chill in the refrigerator.
Press the remaining dough evenly in the bottom of the prepared pan. Bake the crust for 12-15 minutes, until lightly golden. Transfer to a wire rack and let cool while you make the blackberry layer.
In a medium bowl, toss together the blackberries, sugar, salt, cornstarch and lemon zest/juice to combine.
Spoon the blackberry mixture of the crust and crumble the remaining dough over the blackberries.
Bake for 40-45 minutes, until the topping is golden but not burnt. The blackberry filling should be slightly bubbly.
Cool completely before cutting into squares and serving. I usually cut off the edges as they will be very firm and chewy. Store leftovers in an airtight container. They also freeze well.