Prep the pan and oven. Preheat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment paper, leaving a little overhang for easy removal. Lightly grease the sides.

Mix dry ingredients. In a large bowl, whisk together almond flour, coconut flour, sweetener, baking powder, and salt until well combined and lump-free.

Whisk the wet ingredients. In a separate bowl, whisk the eggs, almond milk, yogurt or sour cream, melted butter, lemon zest, lemon juice, and vanilla until smooth.

Combine. Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined. The batter will be thick but spreadable.

Fold in blueberries. Gently fold in the blueberries. If using frozen berries, do not thaw; toss them in a teaspoon of almond flour first to reduce streaking.

Fill the pan. Spread the batter evenly into the prepared loaf pan. Smooth the top and tap the pan lightly on the counter to release any air pockets.

Bake. Bake for 45–55 minutes, or until the top is golden and a toothpick inserted in the center comes out mostly clean (a few moist crumbs are fine). If the top browns too quickly, tent loosely with foil for the last 10–15 minutes.

Cool completely. Let the loaf cool in the pan for 15 minutes, then lift out using the parchment. Transfer to a rack and cool fully before slicing. Keto loaves firm up as they cool; slicing too early can cause crumbling.

Optional glaze. Whisk powdered sweetener with lemon juice and a pinch of salt until smooth. Drizzle over the cooled loaf, then let it set for 10–15 minutes.