One of the most compelling reasons to pick your own blackberries is that you can snag a few tart, just-underripe berries for making jelly without added commercial pectin. Store-bought or even prepicked produce is likely to be all ripe, sweet, and not as good for jelly making. The jelly's even better made with wild berries, whose seeds add a touch of bitterness to the juice that I think pleasantly offsets the sweetness.
- 4 pounds blackberries, preferably wild, some of them tart and not quite ripe, gently rinsed
- About 3 cups sugar
- 3 tablespoons strained fresh lemon juice
Put the blackberries in a wide, 6- to a-quart preserving pan and crush them with your hands. Add ½ cup water and bring to a boil; boil until the berries are tender and have released their juices, about 5 minutes.
Set a large, very-fine-mesh sieve (or jelly bag) over a deep bowl or pot. Pour the blackberries into the sieve and let drain for 30 minutes, stirring occasionally but not pressing down too hard on the solids; discard the solids. Measure the juice; you should have about 4 cups.
Prepare for water-bath canning: Sterilize the jars and keep them hot in the canning pot, put a small plate in the freezer, and put the flat lids in a heatproof bowl (see Notes for details).
Rinse the preserving pan and pour in the blackberry juice. Add ¾ cup sugar for each cup of juice, then add the lemon juice. Bring to a boil over high heat and cook, stirring occasionally, until the mixture registers about 220°F on a candy thermometer or a small dab of it spooned onto the chilled plate and returned to the freezer for a minute wrinkles when you nudge it, about 20 minutes.
Ladle boiling water from the canning pot into the bowl with the lids. Using a jar lifter, remove the sterilized jars from the canning pot, carefully pouring the water from each one back into the pot, and place them upright on a folded towel. Drain the water off the jar lids.
Ladle the hot jelly into the jars, leaving ¼ inch headspace at the top. Use a damp paper towel to wipe the rims of the jars, then put a flat lid and ring on each jar, adjusting the ring so that it's just finger-tight. Return the jars to the water in the canning pot, making sure the water covers the jars by at least 1 inch. Bring to a boil, and boil for 5 minutes to process. Remove the jars to a folded towel and do not disturb for 12 hours. After 1 hour, check that the lids have sealed by pressing down on the center of each; if it can be pushed down, it hasn't sealed, and the jar should be refrigerated immediately. Label the sealed jars and store.