Prepare a boiling water canner, along with jars and lids.

Add blackberries and water to a large stainless-steel saucepan. Bring to a boil, stirring frequently.

Reduce heat, cover loosely, and simmer gently 5 to 10 minutes, stirring and crushing occasionally, just until berries are softened.

Transfer the hot berry mixture to a dampened jelly bag or a cheesecloth-lined strainer set over a deep bowl. Let drip undisturbed until it stops dripping.

Optional for clearer juice: refrigerate strained juice overnight to let sediment settle. Carefully pour off the clear juice and discard sediment.

Pour juice into a saucepan. Add sugar to taste if desired. Bring juice to a gentle simmer.

Ladle hot juice into hot jars, leaving 1/4 inch headspace. Wipe rims, apply lids, and tighten bands to fingertip tight.

Process in a boiling-water canner for 10 minutes, or 15 minutes above 6,000 feet in elevation.

Turn off heat, remove canner lid, and let jars rest 5 minutes. Remove jars and cool 12 to 24 hours, then check seals.