In a bowl, toss 4 cups of blackberries, sugar, flour, lemon juice, and lemon zest.
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.
Take the tart out of the oven, let it cool for about 30 minutes, and top with toasted slivered almonds.