Pre-heat the oven to 425°F.
Place the pie crust on a baking tray or in a pie dish.
In a medium bowl, add blackberries, ¼ cup flour, and ⅓ cup sugar. Gently mix together and lightly smash some of the blackberries so the flour can help thicken the juice as the pie cooks.
Place berry mixture into the pie crust. Set aside to make the crumble.
In a bowl, combine the butter and the sugar and stir till smooth. Add the cinnamon and the flour.
Use a fork to mix the ingredients and create clumps of crumble.
Spoon crumble over the blackberries.
Place in the oven and bake for 10 minutes. After 10 minutes, reduce oven to 350°F and bake for an additional 40 minutes.
Remove from oven and allow to cool before serving. This make take and hour or 2, but it allows the pie to thicken.