Wash the blackcurrants are and grind them in a meat grinder. Mix them with the water to make a thick paste. You can also use a blender to mash the fruits.
Put the mixture on the stove and heat it to 158°F (70°C) for about 10 minutes. Remove from the heat and leave it to sit overnight. In the morning, strain the juice through a double gauze. Put the juice with the sugar back on the stove to boil.
Distribute the prepared juice into glass bottles, cap and keep in a cool, dark place.