Pour about 2 cups water into a large stock- or soup pot. Add berries and crush them with a potato masher. Bring to a boil and turn off the heat.
Now layer a sieve with cheesecloth, hold over a bowl, add berry mixture, and let the juice drain into the bowl. (Make sure no seeds get through.) This should yield about 3½ cups of juice. (If you need more liquid, add apple juice or water.)
Pour blackberry juice back into the pot. Add lemon juice and pectin and bring to a boil. Add sugar and boil 1 minute. Add butter.
Pour mixture into 4 pint jars. Pour a half-inch layer of paraffin over the top of each jar and seal them.
The cook, Ruth, no longer processes her jars in a hot-water bath, but instead runs them through the dishwasher and takes them out just after the cycle is done, so they’re still hot and sterilized. If your dishwasher gets hot enough, this is a good method; otherwise boil the jars and lids. Ruth also seals her jars with paraffin, eliminating the need for water processing. You may also use the following recipe to make elderberry jelly, if you have a good and trusted source. (Ruth picks hers on the banks of the Androscoggin River.) She calls elderberry the “Cadillac” of jellies and says it makes a great gift.