Given the natural sweetness of fresh blueberries, you can skip the syrup.
Whipping egg whites to stiff peaks requires absolute attention just before incorporating them into the batter. Fat inhibits foam stability completely, meaning any trace of egg yolk in the white will prevent proper volume. Clean your mixing bowl thoroughly with a drop of white vinegar and a paper towel beforehand to remove any lingering grease residue. Beat the white until it holds rigid peaks that stand straight up when the beaters are lifted, signaling sufficient protein restructuring to trap air. Folding this foam gently requires a spatula to cut down the center, sweep around the bowl bottom, and flip over onto itself in three separate additions. Over-mixing deflates the trapped air bubbles instantly, transforming a light batter into a dense paste. Whole fresh blueberries fold in last after the egg whites are fully integrated. Fresh fruit bursts when heated on the iron, releasing moisture directly into the pockets of the waffle grid. Grease the waffle iron grids thoroughly with melted fat or oil spray before every single pour to prevent the delicate batter from tearing upon release. Heat control dictates the final exterior texture. Watch the steam escaping from the edges of the iron closely. Cessation of visible steam indicates that the moisture inside the batter has evaporated and the starches have set against the hot metal plates. Lift the lid only when the steam subsides completely to guarantee a crisp, golden-brown crust rather than a soggy interior. Removing waffles prematurely causes them to tear apart along the grid lines. Cool finished pieces on a wire rack rather than stacking them on a flat plate, because trapped condensation softens the crisp exterior crust within seconds.