Preheat the oven to 425℉. Generously grease the popover tin with 1 tablespoon of melted butter. Set aside.
Using an immersion blender (or standard blender), mix together the milk and eggs until there are no longer any streaks of whites or yolks. Add the flour and salt, and pulse until combined. Add the remaining 2 tablespoons of melted butter, lemon zest and lemon juice and pulse until the batter is thoroughly incorporated. Alternatively, whisk the batter by hand. Stir in the blueberries.
Fill the greased popover tin ⅔ to ¾ full of batter. Place the tin in the center or bottom third of the oven. Bake for 35 to 40 minutes undisturbed. Do not open to peek at the popovers during this time. The batter needs all the heat and steam from the oven to rise.
The finished popovers should be golden brown on the outside and feel light in weight. They should be crisp on the outside and slightly soft and chewy on the inside. Serve the popovers warm.
Notes
Fresh blueberries work best, but frozen berries can be used without thawing.
Make sure the popover pan is well greased with butter so the popovers release easily.
Do not open the oven door while baking or the popovers may collapse.
Popovers are best served warm when the exterior is crisp.
Leftovers can be reheated in a 350°F oven for 5 minutes to restore crispness.