Preheat the oven to 325°F (163°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting.
Make the crust: In a bowl, mix almond flour, granular sweetener, and salt. Stir in melted butter and vanilla until the mixture looks like damp sand.
Press firmly and evenly into the prepared pan.
Par-bake the crust for 8–10 minutes, until lightly golden at the edges. Set aside to cool slightly.
Make the blueberry swirl: In a small saucepan, combine blueberries, powdered sweetener, and lemon juice. Cook over medium heat for 4–6 minutes, stirring and gently smashing berries as they soften.
If too thick, add a splash of water. Simmer until jammy, then cool.
Make the filling: Beat cream cheese with powdered sweetener until smooth and fluffy, about 1–2 minutes. Mix in sour cream, vanilla, lemon zest, and salt.
Add eggs one at a time, mixing on low just until combined. Do not overmix.
Assemble: Pour the cheesecake filling over the crust and smooth the top. Dollop spoonfuls of blueberry mixture over the surface and use a knife or skewer to create gentle swirls.
Bake for 23–28 minutes.
The edges should be set and the center slightly jiggly, not liquid.
Cool in the pan on a rack, then chill for at least 3 hours (overnight is best). Lift out using the parchment and slice into bars.