Preheat oven to 350F. Line the bottom of a 8-inch pan with removable bottom (I use this one) with parchment paper. Spray with cooking spray over the parchment and the sides of the pan. Set aside.
In a large bowl combine the egg yolks and powdered sweetener, beat on medium-high speed for 3-5 minutes, until pale, thick ad fluffy. Add the lemon zest and almond flour. Mix until just combined.
In a separate bowl beat egg whites on medium-high speed for 2-3 minutes until stiff peaks.
Fold 1/4 of the egg whites into the almond egg yolk mixture, it will be thick. Continue adding a little bit of the egg whites to the almond egg yolk mixture, folding after each addition. Alternatively (not recommended, but could work), incorporate the egg whites into the mixture using a mixer on the lowest setting and stop beating until just combined. Fold in the blueberries.
Pour the batter into the pan, smooth the top. Bake for 30 minutes, until toothpick inserted comes out clean. Let the cake cool inside the pan for 15 minutes, then remove. Dust with powdered sweetener, top with blueberries, cut and serve.