Rinse the currants, then drain them. Sort the blackberries. Bring both to a boil with 200 ml of water and cook until the berries burst. Place on a juice towel and drain well, then squeeze the towel out additionally.
Measure out 750 ml of juice and mix with the preserving sugar and lemon juice in a large saucepan. Bring to the boil while stirring and let it simmer for 4 minutes.
Stir in the liqueur and fill the jelly to the brim in glasses that have been rinsed with hot water.