Grease a 9-inch springform pan and line the base with parchment paper for smooth release. Double-wrap the outside in heavy foil to prevent water ingress in the water bath. Place a rack in the center of your oven and preheat to 325°F (163°C).

Pulse graham crackers into fine crumbs in a food processor. Mix with melted butter until the mixture forms damp sand and clumps when pressed. Press evenly into the pan base using a flat-bottomed glass, pushing the crumbs 1 inch up the sides. Bake crust 10 minutes, then cool to room temperature.

Combine blueberries, sugar, and 1 tablespoon lemon juice in a medium saucepan. Bring to a simmer over medium heat, stirring occasionally. Mash some berries with a fork as the mixture thickens and deepens in color, about 6 minutes. Stir in cornstarch slurry and cook 1 minute, until glossy and thickened. Remove 2/3 cup mixture, pressing it through a fine mesh strainer into a clean bowl for the swirl; refrigerate the rest for topping.

Beat softened cream cheese and sugar in a stand mixer fitted with paddle attachment until completely smooth, 2-3 minutes, scraping bowl midway. Beat in sour cream, lemon zest, remaining lemon juice, vanilla, and salt until just blended, then add eggs one at a time at low speed. Mix only until smooth and glossy; avoid overbeating to prevent excess air bubbles.

Pour half the cheesecake batter onto the cooled crust. Dollop strained blueberry mixture in several spoonfuls across batter and gently swirl with a skewer or small knife; add remaining batter and repeat swirl with more blueberry. Drag a clean skewer through just the surface, avoid overmixing, or the filling color will muddy.

Place the cheesecake pan in a large roasting pan and pour in boiling water to come halfway up the springform’s sides. Transfer carefully to the oven. Bake 55-65 minutes; the edges should be set but the center jiggles slightly when gently shaken. Do not bake until fully firm, carryover heat will finish setting.

Turn off oven, prop door open several inches, and let cheesecake cool in the oven for 1 hour. Carefully remove water bath and foil, then cool cheesecake to room temperature. Cover and refrigerate for at least 6 hours, preferably overnight, to completely set and develop flavor.

Just before serving, spoon reserved blueberry topping evenly over the chilled cheesecake. For a glossy finish, apply with a small offset spatula and garnish with a few fresh blueberries if desired. Use a hot, dry knife for clean slices, wiping blade between cuts. Serve cold for best texture.