Whisk 60 g sugar and 8 g cornstarch together in a small saucepan, then add the blueberries, water, and lemon juice. Cook over medium heat for 7 to 9 minutes, stirring and crushing roughly half the berries with a silicone spatula. The mixture is ready when it looks glossy, bubbles slowly, and leaves a brief trail when scraped across the pan. Transfer it to a shallow bowl and cool to room temperature; hot fruit would soften the filling.
Set an oven rack in the middle position and preheat the oven to 350°F. Lightly grease a 9-inch square metal pan with sides at least 2 inches deep. Line it with two crossed strips of parchment, leaving enough overhang to lift the chilled cheesecake out later. Press the parchment neatly into the corners so folds do not distort the bars. Avoid a shallow pan because the filling can approach the rim during baking.
Combine the graham cracker crumbs, 30 g sugar, and 1/4 teaspoon salt in a medium bowl. Stir in the melted butter until every crumb is evenly moistened and the mixture holds when squeezed. Scatter it across the prepared pan, then compress it into a level layer using the flat bottom of a measuring cup. Bake at 350°F for 10 minutes, until fragrant and slightly darker at the edges. Cool on a wire rack while lowering the oven to 325°F.
Place the softened cream cheese in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium-low speed for about 1 minute, stopping to scrape the bowl and paddle, until smooth and free of firm lumps. Whisk 150 g sugar, 15 g cornstarch, and 1/4 teaspoon salt together, then add them to the cream cheese. Mix on low for 1 to 2 minutes, scraping twice, just until creamy and uniform rather than fluffy.
Add the room-temperature sour cream, vanilla, and lemon zest to the cream cheese mixture. Blend on low speed until smooth, then scrape the bottom and sides of the bowl thoroughly. Add the eggs one at a time, mixing on low only until each yolk and white disappear before adding the next. After the final egg, fold the batter several times by hand. The finished filling should flow thickly from the spatula with few visible air bubbles.
Pour about half the cheesecake filling over the cooled crust and spread it into the corners with an offset spatula. Spoon half the cooled blueberry mixture over the surface in small, separated dollops. Cover with the remaining filling, spreading gently so the layers stay distinct, then add the remaining blueberry mixture in teaspoon-size patches. Drag a skewer or thin knife through the top in a loose figure-eight pattern only a few times; excessive swirling muddies the colors and weakens clean berry ribbons.
Bake at 325°F for 42 to 48 minutes. The outer 2 inches should look set and slightly puffed, while the center should wobble together like soft gelatin when the pan is nudged, not ripple like liquid batter. An instant-read thermometer inserted near the center should register about 150 to 155°F. Turn off the oven, prop the door open with a wooden spoon, and leave the cheesecake inside for 20 minutes to begin cooling gradually.
Transfer the pan to a wire rack and cool for about 1 hour 40 minutes, until it no longer feels warm. Cover without touching the surface and refrigerate for at least 6 hours, preferably overnight. Lift the firm cheesecake out by the parchment and place it on a cutting board. For 16 clean bars, trim only visibly uneven edges, then make four equal cuts in each direction using a long knife warmed under hot water and wiped completely dry before every cut.
