Prepare the Crust:
In a medium bowl, combine graham cracker crumbs, sugar, and cinnamon (if using). Add melted butter and mix until the crumbs are evenly coated. Press the mixture into the bottom of a springform pan and refrigerate while you prepare the filling.

Make the Blackberry Puree:
Place fresh blackberries in a food processor. Puree until smooth, then strain through a fine mesh sieve to remove the seeds. Stir in 1/4 cup sugar and water. Set aside.

Make the Cheesecake Filling:
Beat the cream cheese and sugar in a large bowl until smooth and fluffy. Add sour cream and vanilla extract and mix until fully combined. Add the eggs one at a time, beating well after each addition until the mixture is smooth and creamy.

Assemble the Cheesecake:
Pour half of the cheesecake filling over the prepared crust. Drizzle half of the blackberry puree on top. Use a butter knife to gently swirl the puree into the cheesecake filling. Repeat with the remaining filling and puree.

Bake the Cheesecake:
Preheat your oven to 325°F (165°C). Bake the cheesecake for 55-65 minutes, or until the edges are set, but the center still has a slight jiggle. Turn off the oven and leave the cheesecake in the oven with the door slightly open for 10 minutes. Then, remove from the oven and let cool to room temperature.

Chill and Serve:
Once the cheesecake has cooled, cover and refrigerate for at least 4 hours, or overnight. Top with additional blackberries before serving.