Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving enough overhang on two sides to lift out the chilled bars later.

Combine the almond flour, powdered sweetener, and fine salt in a bowl. Stir in the melted butter until the mixture looks like damp crumbs and holds together when pressed.

Press the crust mixture firmly and evenly into the prepared pan. Bake for 10–12 minutes, just until the edges become lightly golden.

Whisk the eggs, granulated allulose or powdered erythritol, lemon juice, lemon zest, coconut flour, and salt until smooth. Let the mixture stand for 2–3 minutes so the coconut flour can hydrate, then whisk once more.

Scatter the fresh blueberries over the warm crust. Pour the lemon filling evenly over the berries, taking care not to disturb the crust.

Bake for another 20–26 minutes, until the center is set with only a slight jiggle. Avoid overbaking because the custard continues to firm as it cools.

Cool the pan completely at room temperature, then refrigerate the bars for at least 2 hours. Lift them out using the parchment overhang, dust with powdered erythritol if desired, and cut into 12 bars.