This recipe makes a simple blackcurrant jam that's just sweet enough and plays up the spicy flavor of the blackcurrants. One batch yields 8 half-pint jars, but can easily be divided in half.
Add the blackcurrants to a thick-bottomed pot and simmer on low for about 10 minutes until they begin to release their juice.
Add sugar and lemon juice (if using). Simmer for about 30 minutes, stirring frequently to prevent boiling over. It will foam heavily, make sure the pot is no more than 2/3rds full when you start.
When the jam stops foaming and begins to thicken, cook for another few minutes before pouring into canning jars. If you're using a thermometer, the jam should be roughly 218 to 220 degrees F at sea level. Be sure to adjust for altitude! See note above.
Load into half-pint canning jars leaving 1/4 inch headspace. Attach 2 part lids and process in a water bath canner for 10 minutes. Or, alternatively, store in the refrigerator and use within a month or two.