In a bowl, toss 4 cups of blackberries, 1 cup sugar, ½ cup flour, 2 tablespoons lemon juice, and 1 tablespoon lemon zest. Combine thoroughly.
How to assemble blackberry tart
Spoon this mixture into the cooled tart shell. Top with the remaining 1 cup of blackberries. Sprinkle 1 or 2 tablespoons white sugar over the berries, and then sprinkle thinly chopped cold butter on top, too.
Place the tart on a rimmed baking sheet (to catch any juices) and bake for 30-40 minutes at 350 F until filling starts to bubble. If crust starts to brown too much, place a piece of aluminum foil over the tart.