Preheat your oven to 160ºC (325ºF). In a food processor, blend the digestive or graham crackers with granulated sugar until fine crumbs form. Mix in melted butter until it resembles wet sand. Press the mixture into the bottom of a lined 9-inch springform pan and bake for 10 minutes. Let it cool completely.

In a bowl, gently toss fresh blueberries with sugar, flour, and lemon juice to coat them evenly. For the crumble topping, combine flour and dark brown sugar in another bowl, then mix in melted butter until you achieve a crumble texture.

In a large mixing bowl, beat the cream cheese until smooth and creamy. Gradually add granulated sugar and mix until well combined. Whisk in sour cream, cornstarch, and vanilla extract. Incorporate the eggs two at a time, mixing well after each.

Pour the cheesecake filling into the cooled crust and spread it evenly. Layer the prepared blueberries on top, then sprinkle the crumble mixture. Bake in a water bath for about 1 hour and 20-30 minutes, until slightly wobbly in the center.

Turn off the oven, crack the door slightly, and let the cheesecake cool inside for 1 hour. Transfer to a wire rack to cool for another hour, then refrigerate for at least 6 hours.