Give the berries a good wash and rinse, remove the tiny stems from the berries, and discard blemished ones.
Put clean blackcurrant berries in a saucepan, cover them with a small amount of water, and cook until it comes to a boil and the berries start bursting.
With a spatula, spoon, or potato masher crush the berries to release the juice.
Add the rest of the water, and sugar and cook until the sugar dissolves and starts boiling.
You will notice a foam forming on the surface.
With a slotted spoon skim off the foam and discard.
Cook until the syrup reaches the soft ball stage at 240°F or 116°C on a thermometer (or put a drop into cold water and see if it turns into a soft pliable ball.)
Add the lemon juice, give it another stir, and turn the heat off.
Pour the hot jam into clean and sterilized jars. For prolonged storage use your preferred canning method (see the notes above).
The provided nutritional information is an estimate per serving and is not guaranteed for accuracy.