Preheat the oven to 350°F and line an 8x8 inch baking pan with parchment paper, leaving an overhang. Mix the almond flour, powdered sweetener, lemon zest, salt and melted butter until clumpy.
Press two thirds of the mixture firmly into the pan and bake for 10 minutes. Toss the remaining crumbs with the sliced almonds and set aside, then reduce the oven to 325°F.
Simmer the blueberries, powdered sweetener and lemon juice in a small saucepan over medium heat for 5 minutes, mashing lightly, until jammy. Whisk in the xanthan gum and let the sauce cool.
Beat the cream cheese and powdered sweetener until smooth. Mix in the sour cream, lemon juice, lemon zest and vanilla, then add the eggs one at a time on low speed just until combined.
Pour the lemon filling over the crust. Dollop the blueberry sauce on top and drag a toothpick through it to create purple swirls.
Sprinkle the reserved almond crumbs evenly over the top.
Bake for 32 to 36 minutes, until the crumbs are golden and the center jiggles just slightly.
Cool for 1 hour at room temperature, chill for at least 4 hours, then lift out using the parchment and slice into 16 bars.