Place blackcurrants in a large pot or a salad spinner. Fill with water and let them soak for 5 minutes to loosen debris and leaves. Rinse thoroughly until the water runs clear.

Remove stems and blossom ends from the blackcurrants.

Wash the blueberries and mix them with the destemmed, washed blackcurrants.

Place blackcurrants and blueberries in a pot and add one pound (450 g.) of sugar. Reserve the rest of the sugar to be mixed with pectin and added later.

Mash the berries with a potato masher.

Cover and let the mashed berries sit for at least 8 hours or overnight to let the juices come out.

Once rested, put mashed berries together with their rendered juices into a food processor or blender and crush.

Bring the jam mixture to a boil over medium heat, stirring often to dissolve sugar.

Skim off any foam from the surface.

Reduce the heat to medium-low and cook the jam mixture for 25 minutes.

Add reserved sugar-pectin mixture to the pot.

Stir well, then cook for another 10 minutes. If you're not sure if the jam is thick enough, do a chilled plate test. Cook for another 5 minutes if you want it thicker.

Pour the hot jam into clean, sterilized canning jars.

Seal the jars using your preferred canning method.