Preheat the oven to 350°F (175°C) and line an 8x8-inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.

In a medium bowl, combine the almond flour, melted butter, 0.33 cup of the sweetener, and salt. Mix until a crumbly dough forms.

Press the crust mixture firmly into the bottom of the prepared pan and bake for 10 minutes until the edges are lightly golden. Remove from the oven and let cool slightly.

In a small saucepan over medium heat, combine the blueberries, 1 tablespoon of sweetener, and water. Simmer for 5-8 minutes, mashing the berries with a fork as they soften, until the mixture reduces into a thick sauce. Strain through a fine-mesh sieve if a smooth syrup is desired.

In a large mixing bowl, beat the softened cream cheese and the remaining 0.5 cup of sweetener with an electric mixer until completely smooth and fluffy.

Add the eggs one at a time, beating on low speed until just incorporated. Stir in the lemon juice and vanilla extract, being careful not to over-aerate the batter.

Pour the cheesecake filling over the par-baked crust and spread it into an even layer with a spatula.

Drop small spoonfuls of the blueberry sauce onto the surface of the cheesecake batter. Use a toothpick or skewer to gently swirl the sauce into the batter to create a marble effect.

Bake for 25 to 30 minutes, or until the edges are set and the center has a slight jiggle when the pan is moved.

Allow the bars to cool to room temperature in the pan, then refrigerate for at least 4 hours (preferably overnight) before slicing into 12 even squares.