Clean berries just before using. Add the blackberries and lemon juice to an extra-large saucepan. Mash with a potato masher and simmer for a few minutes, to break down the fruit.

Use a food mill, or press mixture through a fine mesh strainer, into a bowl, to remove seeds.

Measure out 4 cups of berry juice and add to an extra-large heavy bottomed stock pot. Add 2-3 spoonfuls of the leftover seeds for blackberry "jam".

Add sugar and stir to combine. Turn burner to medium low, stirring occasionally, cooking for several minutes until sugar has dissolved.

Increase the heat to medium high, and cook, stirring constantly, until the mixture comes to a full boil (a rolling boil that can't be stirred down).

Add the pouch of pectin, stirring continuously, and allow to return to a full boil. Set a timer for 1 minute, stirring continuously, and remove from the heat after 1 minute.

Pour jam into prepared jars and seal with lids.