Put a loaf pan in the oven and preheat the oven to 180C/350F.
Whisk(electric mixer) the egg whites in a clean mixing dish. Give this some time—about two minutes. Set aside.
Add the cooled, melted butter and egg yolks to a second, sizable mixing bowl. (I microwaved the butter for 1 minute, then let it cool for 2 minutes.) Use an electric mixer to blend while whisking. Incorporate lemon juice and sour cream.
In a third mixing bowl, combine all of the dry ingredients. Lemon zest should also be added.
Combine 1/2 of the stiff egg whites with the dry ingredients and add them to the yolk mixture. Mix.
Smoothly fold(spatula) the remaining 1/2 of the egg whites into the batter. Don't over-mix or the eggs may deflate. Add the blueberries last, then fold.
Pour(spatula) the batter into the loaf pan. Level.
On the middle shelf, bake for 65 to 70 minutes, or until golden and a spear can be inserted and removed without crumbs sticking. After 40 minutes, check the keto blueberry bread. Loosely cover it with aluminum foil.