Place the zucchini in a sieve and sprinkle with the salt. Toss to combine. Let drain 1 hour and then squeeze out the zucchini to release as much moisture as possible.
Preheat the oven to 325F and grease 2 (two) 9-inch round cake pans very well. If your pans are not very non-stick, you may want to line them with parchment paper and grease the parchment.
In a medium bowl, combine the almond flour, coconut flour, whey protein, baking powder and salt.
In a large bowl, beat the butter with sweetener until well combined. Beat in the eggs one at a time, scraping down the sides of the bowl and the beaters as needed. Then beat in the zucchini, lemon zest, and vanilla extract.
Beat in the almond flour mixture until well combined, then beat in the lemon juice. If your batter is quite thick, add a little water, 1 tbsp at a time, until a spreadable consistency is achieved.
Gently fold in the blueberries. Divide the batter between prepared pans and spread evenly to the edges. Bake 30 to 40 minutes, until golden brown around the edges and the top is set to the touch.
Remove and let cool completely in the pans, then run a sharp knife around the edges to loosen.
In a large bowl, beat the cream cheese and butter together until smooth. Beat in the sweetener until well combined, then beat in the lemon zest.
Beat in the lemon juice and cream.
Place one cake layer on a serving platter and spread the top with half of the frosting. Add the second cake layer and spread with the remaining frosting. Top with fresh blueberries.