Combine graham cracker crumbs and granulated sugar in a mixing bowl (or right in a 9x13 baking dish to reduce dishes).
Melt butter in a heat-proof bowl in the microwave. Pour melted butter into the graham cracker mixture and stir until evenly combined.
Press graham cracker mixture evenly across the bottom of a 9x13 baking dish. Place baking dish in the freezer to firm up the crust while making the cheesecake mixture.
In a medium mixing bowl, beat together cream cheese, 1 ½ cups of powdered sugar, vanilla extract, and sour cream until thoroughly combined.
Pour 2 cups of heavy whipping cream into a chilled mixing bowl. Beat on medium high speed (either with a standing mixer whisk attachment or with a handheld mixer). Continue beating until whipped cream forms stiff peaks.
Gently fold the whipped cream into the cream cheese mixture.
Spread the cheesecake filling evenly over the chilled graham cracker crust. Use an offset spatula to smooth the top of the cheesecake.
Place cheesecake in the refrigerator for at least 4 hours or in the freezer for at least 2 hours to set.
While cheesecake is chilling, prepare the blackberry topping.
Place blackberries, 1 ½ cups sugar, 2 teaspoons vanilla extract, ¼ teaspoon cinnamon, water, and cornstarch in a medium saucepan.
Bring mixture to a boil, stirring constantly. Reduce heat and simmer for 8-10 minutes, stirring occasionally, until blackberries are broken down and juices have thickened to a jam-like consistency.
Remove from heat and let cool completely.
Spread cooled blackberry topping over the chilled cheesecake. Cut into pieces and serve.