In a large pot over medium‑high heat, combine the blackberries and water. Bring to a boil, then use a potato masher or large spoon to gently crush the berries as they heat. Let the mixture cook for about 20 minutes until it’s reduced and the berries are soft.

Place a fine‑mesh strainer over a bowl and pour the cooked blackberry mixture through it. Let the juice drain without pressing the solids; this ensures a clear jelly. You should end up with about 3¾ cups of blackberry juice.

In a clean heavy pot, combine the strained blackberry juice and powdered pectin. Stir well to dissolve the pectin. If using, add the butter to help minimize surface foam.

Bring the juice and pectin mixture to a full rolling boil that can’t be stirred down.

Add the sugar all at once and stir to combine. Return the mixture to a full rolling boil, and boil hard for exactly 1 minute, stirring constantly. Remove from heat and skim off any foam with a spoon if needed.

Using a ladle, pour the hot jelly into clean jars, leaving about ⅛–¼ inch headspace. Wipe the jar rims with a clean damp cloth, place the lids on top, and seal. Allow jars to cool at room temperature.

If you don’t plan to can the jelly for long‑term storage, refrigerate once cool; it will set and stay good in the fridge for a few weeks.