Wash half pint jelly jars and lids (or whatever size you choose) with hot soapy water and let dry on a kitchen towel.
Combine the blackberries and water in an 8 quart saucepan or pot. It needs to be at least 8 quarts because the mixture rises when boiling. Bring the mixture to a boil. Reduce heat and simmer 2 minutes.
While blackberries are cooking, seed and mince the jalapeno peppers. Cut off the stem end of the jalapeno peppers and cut them in half. With a sharp paring knife, make lengthwise cuts on either side of the seeds to remove them and the membrane. After removing the seeds, cut the pepper halves into thin strips. Hold a few strips together with one hand, and cut into tiny pieces with a chef's knife with the other hand. Mash the blackberries in the pan with a potato masher or a large spoon. Strain the cooked blackberries through a large fine metal sieve or strainer in to a bowl.
Press the back of a large spoon against the strainer to help mash the juice through the strainer. The goal is to get 2 cups of juice. See NOTES if you don't get 2 cups.
Combine the blackberry juice, minced jalapeno peppers, sugar and vinegar in the same 8 quart pot. Bring to a rolling boil over medium-high heat, slowly stirring often, and boil 3 full minutes. Set a timer.
Quickly stir in the liquid pectin and bring back to a boil, stirring constantly. Bring back to a rolling boil that can't be stirring down and boil 1 minute. Remove from heat and skim any foam that collects on top.
Pour hot jelly into jars using a canning funnel if you have one. Wipe top of jars clean with wet paper towel and seal jars with lids. Let stand on a kitchen towel at room temperature for 24 to completely set up. I usually can't resist slathering some on a piece of toast if I have a tiny bit that won't fit in the jars!
Label the jars with stickers or write on the lid. Store Freezer Blackberry Pepper Jelly in the refrigerator for 3 weeks or in the freezer for up to 1 year.