Preheat the waffle iron to medium and lightly grease with a little neutral oil.
Whisk flour, sugar, baking powder, and salt in a large bowl.
In a second bowl, whisk milk, eggs, and vanilla until smooth.
Stream in the melted butter while whisking the wet mixture.
Pour wet ingredients into dry and stir just until combined; a few small lumps are fine.
Fold in blueberries and lemon zest gently so the batter stays light.
Let the batter rest 5 minutes to thicken slightly.
Cook waffles: add batter to the hot iron, close, and cook until deep golden and crisp, 3–5 minutes. Repeat with remaining batter.
Keep finished waffles warm on a rack in a 200°F oven and serve with maple syrup and extra berries.