Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners. Lightly spray the liners if you want extra insurance against sticking.

In a large bowl, whisk 2 cups almond flour, 2 tablespoons coconut flour, 2 teaspoons baking powder, 1/4 teaspoon salt, and 1/2 teaspoon cinnamon (if using). Break up any lumps in the almond flour.

In a separate bowl, whisk 3 large eggs, 1/2 cup granulated keto sweetener, 1/2 cup sour cream, 1/4 cup melted butter, and 1 1/2 teaspoons vanilla extract until smooth. Add 1 teaspoon lemon zest if you like a hint of citrus.

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

Fold in 3/4 cup blueberries. If using frozen blueberries, do not thaw; toss them in a teaspoon of almond flour first to minimize bleeding.

Divide the batter evenly among the muffin cups. They should be about 3/4 full. For a bakery look, add a few extra blueberries on top.

Bake for 18–22 minutes, or until the tops are set and a toothpick comes out mostly clean with a few moist crumbs. Avoid overbaking to keep them tender.

Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Keto muffins firm up as they cool, so patience helps.