Line a 9×9-inch (or 8×8-inch) square pan with parchment paper, letting it overhang two sides to create handles. Lightly grease the parchment with a little butter or nonstick spray.

If using a crust, stir together 1/2 cup graham cracker crumbs and 1–2 tablespoons melted butter until evenly moistened. Press firmly into the bottom of the lined pan. Chill the pan while you prepare the fudge.

In a small saucepan, combine the blueberries, granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the berries burst and release their juices, about 5–7 minutes. In a small bowl, stir together the cornstarch and cold water to form a smooth slurry, then stir it into the blueberry mixture. Cook 1–2 minutes more, until thickened and glossy, like a loose jam. Remove from heat and let cool completely to room temperature.

In a medium mixing bowl, beat the softened cream cheese with a hand mixer until smooth and fluffy, 1–2 minutes. Add the softened butter and beat until fully combined and silky. Mix in the vanilla and salt. Gradually add the sifted powdered sugar, about 1/2 cup at a time, beating on low at first, until you have a thick, smooth, frosting-like mixture.

Place the white chocolate chips in a microwave-safe bowl. Microwave in 20–30 second bursts, stirring well after each burst, until just melted and smooth. Stop while a few small pieces remain and stir until they melt completely to avoid overheating. Alternatively, melt gently in a heatproof bowl set over a pot of barely simmering water, stirring frequently.

With the mixer on low, slowly pour the melted white chocolate into the cream cheese mixture. Beat until fully blended, thick, and silky, scraping down the sides and bottom of the bowl so no streaks remain.

Spread about two-thirds of the fudge mixture evenly into the prepared pan, smoothing the top. Spoon about half of the cooled blueberry sauce in dollops over the surface. Add the remaining fudge mixture in small spoonfuls between the blueberry dollops. Using a thin knife or skewer, gently swirl the blueberry sauce and fudge together with a few figure-eight motions, being careful not to overmix. If desired, sprinkle a light layer of graham cracker crumbs over the top for extra cheesecake crunch.

Cover the pan loosely with plastic wrap or foil and refrigerate for at least 4 hours, or until the fudge is very firm. For faster setting, transfer to the freezer for the last 30–45 minutes, but do not let it freeze solid for long or it will be harder to slice.

Use the parchment handles to lift the fudge out of the pan onto a cutting board. With a sharp knife, cut into 1–1 1/2-inch squares, wiping the blade between cuts for clean slices that show the blueberry swirls. Serve chilled or slightly cool; keep refrigerated as it softens if left out too long.