(If you are planning to can the jam, prepare waterbath and sterilize jars).
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.
(Optional, but recommended:) 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”. (Jelly uses the juice only).
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.
For Freezer Blackberry Jam: Allow the jam to cool at room temperature for 24 hours, then store in the fridge for up to 1 month, or the freezer for up to 1 year.
To Can: Process in a boiling water bath for 10 minutes (or longer if at high altitude*). Allow to rest on your counter for 24 hours, to ensure the jars settle and seal properly. Check seals, and store jam in a cool, dry place for up to one year.
Yield: makes about 8-9 cups of jam.No-Pectin Blackberry Jam: I like to add pectin to Jam because it helps to thicken it, but if you want to make a no-pectin blackberry jam, you can leave it out! Blackberries have a very high natural pectin. Cook until the jam thickens to a gel stage, 105°C/221°F on an instant read thermometer. Test it by spooning some hot jam onto a chilled cold plate. If the jam jells up (and doesn’t easily slide around) it’s ready.Seedless Blackberry Jam: Use a food mill or fine mesh strainer to remove all seeds, and don't add any back in (as suggested in step 4).High Altitude Increase Processing Time: