Make the blueberry sauce:
Mix the sugar and water in a small pan. Heat until sugar is dissolved, then boil for 3 minutes or until the mixture is thickened, do not let it caramelized. Cool slightly. Puree the fresh blueberry in a food processor or a blender then press through a sieve over a bowl. Stir in the syrup and chill well.
Make the gelato:
Put the lemon juice in a bowl, mash the bananas with fork into the bowl. It is okay if the bananas pieces are a bit chunky. Mix banana mixture with gelato base. If using an ice cream machine, follow the manufacturer's instruction. After the churning process is done while the gelato is still partly frozen, drop blueberry sauce by spoon into the gelato. Mix it just enough to get ripple effects into the gelato.