Prick each blueberry with a fork, then layer the blueberries in a clean, dry jar, sprinkling each layer with some of the sugar.
Pour the kirsch over the sugared blueberries. Seal tightly and shake once or twice.
Leave in a cool place and turn the jar upside down every day for 4 days until the sugar has completely dissolved. Label the jar and leave to mature in a cool, dark place for 3–4 weeks before using. The blueberries will keep, unopened, for 6–12 months.