Prep your canning jars according to the canning instructions prior.
Gently wash, stem, and drain the blackberries.
Crush the blackberries with a potato masher, food mill or use a food processor to lightly crush them. For best results, only crush 1 cup at a time. DO NOT PUREE. Jam should have bits of fruit. Sieve all the pulp to remove the seeds.
Measure the exact amount of prepared blackberries (Juice) into a large 6-8 quart saucepan.
Stir in 1 box pectin into the fruit mixture. Add the 1/2 tsp. butter to reduce foaming, if desired.
Over high heat, bring mixture to a full rolling boil, stirring constantly.
Quickly stir in the sugar and return again to a full rolling boil and let boil exactly 1 minute, stirring constantly. Remove from heat and quickly skim of the any excessive foam/bubbles.
One jar at a time, ladle the hot prepared jam into the hot, sterilized jars. Fill to within 1/8 inch of top of the jar. Wipe rim of jar clean. Immediately place a hot prepared lid and ring on and tighten.
Once jars are filled, Place jars on the elevated canner rack. Lower rack into the canner with the hot water. Add more water if needed, water needs to be at least 1 inch above the jars.
Cover with a lid, and boil for 10 minutes. After 10 minutes of boiling, remove jars with the jar lifter and set on a towel to cool for 12-24 hours.
**NOTE** Jam can only be made in small batches at a time (about 6 jars at a time). DO NOT increase the recipe or the jam will not set up or thicken.