Pour 300ml/1¼ cups water into a saucepan and add the sugar. Place over a low heat until the sugar has dissolved. Bring to the boil and boil rapidly for 10 minutes, then set the syrup aside to cool.
Meanwhile, cook the blackcurrants in a saucepan with 30ml/2 tbsp water over a low heat for 5-7 minutes until tender.
Press the cooked blackcurrants and their juice through a sieve placed over a jug. Stir the resulting blackcurrant puree into the syrup with the lemon juice and the blackcurrant liqueur, if using. Allow to cool completely, then chill for 1 hour.
Pour the chilled blackcurrant syrup into a freezeproof bowl; freeze until slushy, removing and whisking occasionally until it reaches this point. Whisk the egg whites, if using, in a grease-free bowl until they form soft peaks, then gently fold into the semi frozen blackcurrant mixture.
Freeze the mixture again until firm, then spoon into a food processor or blender and process. Alternately freeze and process or blend until completely smooth. Serve the sorbet straight from the freezer.