Add the blackcurrants and half the caster sugar to a small saucepan, place over a low-medium heat and bring to a simmer. Continue to simmer for 10 minutes, or until the sugar has dissolved and the blackcurrants have softened to a pulp. Cover and allow to cool.
Once cool, add the blackcurrants to a blender and blitz until smooth, then pass through a sieve (strai