Clean and remove the stalks of the black currants.
Crush: Place them in a large pot and add the agave syrup. Use a potato masher to crush the currants a little. You don't want a berry puree; mash the currants partially to obtain some juice while retaining most of them intact.
Steep: Mix well and let stand for about an hour. The mixture will become juicier during this time.
Cook: Bring everything to a boil and let cook for about 5 minutes while stirring from time to time.
Thicken: In a small bowl, mix the cornstarch with 2-3 tablespoons cold water to obtain a paste. Slowly pour this mixture into the boiling jam while whisking continuously. Bring to a boil again and cook for one more minute.
Freeze or refrigerate: Let cool, transfer into a clean jar, and refrigerate or freeze.
The nutrition is calculated for the whole jar of jam.