Wash the berries: Rinse blackberries gently, remove stems, leaves, or soft berries, then pat dry. If using frozen berries, thaw them first.
Mash the berries: Add berries to a large bowl or pot and mash with a potato masher. Mash lightly for chunky jam or more thoroughly for a smoother texture.
Strain seeds if desired: For less seedy jam, press part or all of the mashed berries through a fine mesh strainer or food mill.
Add sugar and lemon: Transfer berries to a large heavy-bottomed pot. Stir in sugar and lemon juice until well combined.
Boil the jam: Cook over medium heat until the sugar dissolves, then bring to a rolling boil. Add pectin if using, and boil until thickened.
Skim the foam: Remove any pale foam from the surface with a spoon for a clearer, cleaner-looking jam.
Test the set: Spoon a little jam onto a cold plate. If it wrinkles or holds its shape after 30 seconds, it’s ready.
Fill the jars: Spoon hot jam into clean jars. Cool and refrigerate, freeze with extra space, or process in a water bath if canning.
