Lightly coat donut pan(s) with cooking spray and flour.
In a large bowl, whisk together dry ingredients.
In separate bowl, stir wet ingredients until well-combined. Lightly coat blueberries in flour.
Add wet ingredients to dry, mixing just until combined. (Go gently as over-mixing can lead to dry donuts.) Fold in blueberries.
Fill each donut pan well ¾ full without covering middle section (so you're left with a hole after baking). Be sure to evenly distribute blueberries.
Bake for 12-14 minutes, or until edges are golden brown and top springs back when lightly touched. Allow to cool for 5 minutes in pan, then transfer donuts upside down onto cooling rack. Completely cool before glazing.
For Glaze
In a small bowl, mix powdered sugar with lemon juice. Slowly add milk (a bit at a time) and stir until smooth. Add more powdered sugar if glaze becomes too runny.
Dip bottom of donuts into glaze, then place glazed-side up onto rack to drip dry. Place parchment paper or baking sheet below to catch drippings. Allow to set up, then transfer to airtight container.
Notes
For the gluten-free all-purpose flour, I put together my own using this recipe (with xanthan gum). I have not tested this recipe with regular all-purpose flour, but will update when I do.
To bring eggs to room temperature, place in a bowl with warm water. Allow to sit for about 5 minutes.
Can I use frozen blueberries? You can, but don't thaw. I suggest using fresh. You may need to adjust baking time a bit longer.
The lemon glaze for donuts shouldn't be too thin or it will seep in. Add a bit more powdered sugar if it's runny.
How long will these donuts keep? Store glazed donuts in an airtight container at room temperature for up to 3 days. These donuts are best eaten within a day or two. We like them slightly warmed up. Donuts can be frozen up to 3 months in an airtight container or bag. Allow the glaze to set up first.