1. Macerate the fruit: Combine rhubarb, blackberries, sugar, and lemon juice in a large heavy-bottomed pot. Stir to coat. Cover and leave to macerate at room temperature for at least 1 hour, or overnight in the refrigerator. The sugar will dissolve and the fruit will release deep purple juice. Do not start cooking until the sugar is at least 80 percent dissolved.

2. Bring to a boil and soften: Place pot over medium-high heat uncovered. Stir frequently until the mixture reaches a full rolling boil. Cook 5 to 8 minutes until rhubarb collapses and blackberries burst. Use a potato masher to break down any large rhubarb pieces. For a smooth jam, use an immersion blender for 10 seconds directly in the pot. For a textured jam with visible seeds, skip blending.

3. Low-sugar version only — add Pomona’s pectin: Once fruit is fully soft and at a steady boil, stir in the prepared calcium water, then whisk in the Pomona’s pectin powder. Return to a full rolling boil for exactly 1 minute, stirring constantly. Remove from heat immediately. Skip this step entirely if making the classic version.

4. Classic version — cook to gel point: Continue boiling on medium-high heat, stirring frequently. To test for gel point, drop half a teaspoon of jam onto a frozen plate. Wait 60 seconds, then push the blob with your fingertip. If it wrinkles and holds its shape, the jam is set. If it slides and stays liquid, return to heat and test again in 3-minute intervals. Alternatively, use a digital thermometer and cook to 220°F (104°C) at sea level.

5. Ladle into jars: Remove pot from heat. Skim foam from the surface with a metal spoon. Ladle immediately into warm, sterilized jars. Leave ¼-inch headspace for fridge or water-bath canning jars. Leave ½-inch headspace for freezer jars. Wipe jar rims clean with a damp cloth. Apply lids. For shelf-stable storage: process in a boiling water bath for 10 minutes, adjusted for altitude. For fridge storage: cool to room temperature, then refrigerate.