Sterilize your canning jars in boiling water for 10 minutes.

In a large pot, combine the blackberries and water. Use a potato masher to crush the berries gently.

Pour the crushed berries through a fine mesh strainer or cheesecloth into another bowl to separate the juice from the pulp.

Measure out 4 cups of the blackberry juice.

In the same pot, combine the blackberry juice and lemon juice. Stir in the fruit pectin and bring the mixture to a boil over medium heat.

Once boiling, add the sugar all at once, stirring constantly until it dissolves. Boil the mixture for 1 minute.

To check if the jelly is ready, drop a spoonful on a cold plate and see if it gels. If it doesn't, boil for another minute and test again.

Using a canning funnel, carefully pour the hot jelly into the sterilized jars, leaving about 1/4 inch of headspace. Wipe the rims with a clean cloth.