Note: Reducing sugar, water, pectin or raspberries will result in set failures. USE EXACT AMOUNTS.
Wash the blackberries. Place them all in a thin wire strainer, and mash the berries with a large wooden spoon over and over, this will separate the seeds and will produce a blackberry puree - you will need exactly four cups of puree.
Remove to occasionally scrape the outside bottom of the strainer with the spoon because there is puree on it.
Add the lemon juice to the blackberry puree and stir until combined.
Add exactly 3 cups of sugar, 1 cup of water, and one packet of pectin to a large saucepan.
Bring mixture to a boil on medium high heat, stirring constantly.
Boil and stir for 1 minute. Remove from heat.
Quickly pour the blackberry puree into the hot sugar mixture.
Stir with a large spoon for a few minutes.
Pour the seedless blackberry jam into the clean freezer proof containers, leaving ½ inch space at the top for expansion during freezing;
Cover with lids. Let stand at room temperature for 24 hours until set.
Refrigerate up to 3 weeks. Otherwise, store in the freezer for up to 1 year.