In a large pot, combine the blackberries and lemon juice. Mash the blackberries slightly with a wooden spoon.

Add the sugar and pectin to the pot. Stir well and bring the mixture to a boil over medium heat.

Once boiling, continue to cook for about 10 minutes, stirring frequently.

To check if the jam is ready, place a spoonful on a cold plate and let it sit for a minute. Run your finger through it; if it holds its shape, it's done.

Using a jar funnel, ladle the hot jam into sterilized jars, leaving about ¼ inch of headspace. Wipe the rims clean and seal with lids.

Process the jars in a boiling water bath for 10 minutes to ensure they are sealed properly.