Preheat the oven to 350 degrees F.
For the crust: Line a 9-by-13-inch rectangular baking pan with foil and spray with cooking spray.
Place the vanilla wafers and pecans into the bowl of a food processor and pulse until the mixture becomes crumbs. Add the melted butter and pulse again until combined. Pour the mixture into the prepared pan and press the crumbs into the bottom of the pan. (If they come up the sides, that's okay!)
For the filling: Beat the cream cheese, sugar and vanilla together in a medium bowl with an electric mixer until smooth. Add the eggs one at a time, beating after each addition. Add the sour cream and mix again.
Pour the filling into the crust, smooth the top and bake for 50 minutes. Turn off the oven, open the oven door and allow the pan to sit in the open oven for 15 minutes. Remove and set aside to cool.
For the topping: Add the blackberries, sugar and 1/4 cup water to a saucepan or skillet. Bring to a boil over medium-high heat and cook until the juices thicken slightly, 4 to 5 minutes.
In a small bowl, make a slurry by stirring together the cornstarch and 2 tablespoons water until smooth. Add the slurry to the berries, return to the boil and cook for another 1 to 2 minutes. Turn off the heat and cool the mixture.
Pour the blackberries over the cheesecake and place the pan into the fridge to cool and set for at least 2 hours (several hours is better).
When ready to serve, remove the cheesecake from the pan by lifting the edges of the foil. Slice into 15 pieces with a long serrated knife.