Step 1: Rinse the black raspberries thoroughly and drain any excess water.
Step 2: In a large pot, combine the berries, sugar, and lemon juice. Add the butter if using to prevent foam.
Step 3: Place the pot over medium-low heat and stir constantly until the sugar has completely dissolved into the berry juices.
Step 4: Increase the heat to medium-high and bring the easy black raspberry jam mixture to a full rolling boil.
Step 5: Boil for about 10-15 minutes, stirring frequently, until the mixture reaches 220°F (104°C) or passes the cold plate test.
Step 6: Remove from heat and skim off any remaining foam from the surface of the jam.
Step 7: Ladle the hot jam into sterilized jars, leaving 1/4 inch of headspace. Wipe rims and apply lids.
Step 8: Let the jars cool completely at room temperature before storing in the refrigerator or processing in a water bath for shelf stability.