Begin by sterilizing your jars to ensure a safe and long-lasting blackberry jam. Wash them in hot, soapy water, then place the jars in a pot of boiling water for a few minutes to keep them warm.

Rinse your fresh blackberries thoroughly under cool water and then dry them gently with a towel. In a mixing bowl, lightly crush about 5 cups of the berries using a potato masher or fork. Stir in the measured pectin and fresh lemon juice, ensuring all ingredients blend well together.

Transfer the mashed blackberry mixture into a medium heavy-based pan. Place it over medium heat and bring the mixture to a vigorous boil, stirring continuously to dissolve the pectin completely.

Once boiling, gradually add all of your sugar to the mixture, continuing to stir constantly. Allow it to return to a full rolling boil and cook for exactly 1 minute while stirring.

After boiling, remove the pot from heat and use a spoon to skim off any foam that has formed on the surface of your blackberry jam. Carefully pour the jam into your pre-warmed jars.

If you're looking to preserve your blackberry jam for the long haul, process the jars in a boiling water bath. Submerge the filled jars and let them boil for 10 minutes, then remove the jars and let them cool completely.