In small saucepan over medium-high heat, bring the sugar and 1/2 cup of water to a boil, stir until the sugar dissolves. Boil the syrup for 2 minutes, then remove the pan from the heat and set aside to cool.
Remove the black currants from the stalks by pulling them through the tines of a fork.
In a blender, process the black currants and lemon juice until smooth. ALternatively, chop the black currants coarsely, then add the lemon juice. Mix in the sugar syrup.
Press the puree through a sieve to remove the seeds.
Pour the puree into a non-metallic, freezer proof dish. cover the dish with plastic wrap or a lid, and freeze until the sobet is almost firm, but still a little slushy.
Cut the sorbet into pieces and put into the blender. Process until smooth, then with the machine running, add the egg white and process until well blended. Transfer the sorbet back into the dish, and freeze until almost firm. Chop the sorbet again and process until smooth. ( I know this seems like a lot of processing, but it makes the sobet very smooth, and is really necessary).
Serve immediately ot freeze, tightly covered, for up to 1 week. Let soften 5-10 minutes at room temperature before serving, garnish with mint leaves.