Place the sugar in a saucepan with 175ml (6floz) water and heat gently stirring until the sugar dissolves. Remove from the heat and stir in the orange juice and zest.
Blitz the blackberries in a food processor and push through a nylon sieve to remove the seeds. Stir in the cooled sugar syrup and the cassis.
Pour into a rigid, shallow container and freeze, stirring with a fork every 30 minutes or so until ice crystals start to form and it feels slushy. Alternatively, if you have an ice cream machine, churn the mixture until it starts to freeze.
When the sorbet is slushy, whisk the egg whites until standing on soft peaks. Add to the sorbet and whisk to combine.
Continue to freeze, whisking every 30 minutes or so until completely frozen. If making more than a few days in advance, transfer to an airtight container.
Place the sorbet in the refrigerator for about 30 minutes before serving so that it is soft enough to scoop.