Sort through the blackberries and remove any stems, leaves, or damaged fruit. Rinse gently in cool water and drain. Place the berries in a large pot and crush lightly with a potato masher.

Add the water to the crushed berries, cover, and bring to a boil over high heat. Reduce heat and simmer for 5 minutes until the fruit is completely soft. Pour the hot mixture into a damp jelly bag or cheesecloth-lined strainer over a bowl. Let drip for at least 1 hour until it stops dripping.

Measure 4 cups of strained juice into a large, deep pot. Add 3 cups of sugar (use 3/4 cup sugar per cup of juice if your yield differs). Bring to a boil over high heat, stirring frequently. Boil until the mixture reaches 220°F or sheets from a metal spoon, about 8 to 15 minutes.

Remove from heat and skim off foam. Ladle hot jelly into prepared jars, leaving 1/4 inch headspace. Wipe rims, apply lids and bands to fingertip tight.

Lower filled jars into a prepared water bath canner. Process for 10 minutes for half pints or pints, adjusting for altitude.

Turn off heat and let jars stand in canner for 5 minutes. Remove to a towel-lined counter and cool undisturbed for 12 to 24 hours. Check seals before storing.