Wash blackberries under cool water; if using frozen berries, thaw them completely.

Combine blackberries in a large pot over medium-low heat and simmer for about 10 minutes, mashing occasionally.

Stir in apple cider vinegar or lemon juice and continue mashing. Cook for another 5 minutes.

Simmer for an additional 5-10 minutes until the mixture thickens and becomes syrupy.

Strain the mixture if you prefer a seedless jam.

Measure out blackberry juice and gradually add sugar, stirring constantly until dissolved.

Bring the mixture to a rolling boil and add pectin, stirring continuously for one minute.

Check the consistency by spooning onto a cold plate; if ready, proceed to jar.

Ladle jam into sterilized jars, leaving 1/4 inch headspace, and seal tightly.

Process jars in boiling water bath for 10 minutes or cool completely for freezer storage.