Place a 12-cup muffin tin on the center rack and preheat the oven to 450°F. Let the pan heat for at least 15 minutes.
Meanwhile, in a mixing bowl, whisk the eggs until foamy, about 30 seconds. Whisk in the milk until combined.
Add the flour, sugar, and salt. Whisk just until smooth and no dry flour remains, 30–60 seconds. Let the batter rest while the pan finishes preheating, 10–15 minutes.
Melt the butter if not already melted. Carefully remove the hot muffin tin and quickly brush each cup generously with the melted butter, coating the bottoms and sides.
Gently fold the blueberries into the rested batter just until evenly distributed.
Working swiftly, fill each buttered cup about 2/3 full with batter. Return the pan to the oven and bake at 450°F for 20 minutes without opening the oven door.
Reduce the oven temperature to 350°F (do not open the door) and continue baking until the popovers are tall, deep golden, and feel crisp at the edges, 12–15 minutes more. They should release easily from the pan.
Remove the pan to a rack. Use a paring knife to pierce the side of each popover to vent steam, then tip them out of the tin. Cool 5 minutes and serve warm.