Prebake the 9 inch or 9.5 inch tart crust at 400 Fahrenheit for 20 or 30 minutes according to my super detailed tutorial how to make tart crust shell from scratch. Let the tart shell cool slightly.
How to make blackberry filling
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.