Place the blackberries in a colander and quickly rinse under cool water. Let them drain well so excess water doesn’t dilute the flavor.

Add the rinsed blackberries to a large nonreactive pot. Add just enough water to barely cover the bottom of the pot, about 1/2 cup per quart of berries. Bring to a simmer over medium heat, stirring occasionally. As the berries soften, gently crush them with a potato masher until they are juicy and broken up.

Simmer the crushed berries for 10–15 minutes, stirring occasionally, until they are very soft and have released plenty of juice. Pour the hot mixture through a jelly bag, fine-mesh strainer lined with two layers of cheesecloth, or a nut milk bag set over a large bowl. Let it drip for at least 30 minutes, or up to a few hours. For the clearest jelly, do not squeeze the bag; letting gravity work keeps the juice clear. Measure out 4 cups of strained blackberry juice.

Wash 5–6 half-pint (8 oz) canning jars in hot, soapy water. Rinse well and inspect for chips or cracks. Keep jars hot in a pot of simmering water or a hot dishwasher cycle until ready to fill. Wash lids and bands in warm, soapy water and set aside at room temperature (most modern lids do not need pre-boiling; check package directions). Set up your canning station with a canning funnel, jar lifter, clean damp dishcloth, and a large stockpot or water bath canner with a rack.

In a large, wide pot, combine 4 cups of blackberry juice and 2 tablespoons of bottled lemon juice. Sprinkle the powdered fruit pectin over the surface and whisk to dissolve completely.

Place the pot over high heat and bring the mixture to a full rolling boil, one that does not stop bubbling when you stir. Stir occasionally to prevent scorching.

Once the juice and pectin mixture is at a rolling boil, add all 5 cups of granulated sugar at once, stirring constantly until fully dissolved. Stir in 1–2 teaspoons unsalted butter if using, to help reduce foaming.

Return the mixture to a hard rolling boil over high heat, stirring often. Once it reaches a full rolling boil that cannot be stirred down, begin timing and boil hard for 1–2 minutes, stirring constantly. The jelly will look glossy and slightly thickened. If desired, test for set by placing a small spoonful on a chilled plate, waiting 30 seconds, then running a finger through it; if it wrinkles slightly and holds a line, it’s ready.

Remove the pot from the heat. Skim off any remaining foam from the surface of the jelly with a spoon, if desired. A small amount of foam left on top will not affect flavor or safety.

Working quickly, use a canning funnel to ladle the hot blackberry jelly into hot jars, leaving 1/4 inch of headspace at the top. Run a clean knife or bubble remover tool around the inside of each jar to release any trapped air bubbles. Wipe jar rims with a clean, damp cloth to remove any spills.

Place the lids centered on the jars, then screw on the bands until fingertip-tight—snug, but not overly tight.

Place the filled jars onto the rack in a boiling-water canner, ensuring they are covered by at least 1 inch of water. Return the canner to a gentle boil, then process the jars for 10 minutes, adjusting processing time for altitude according to current USDA guidelines.

Turn off the heat, remove the canner lid, and let the jars rest in the hot water for 5 minutes. Carefully lift the jars out using a jar lifter and place them on a towel-lined surface. Let cool undisturbed for 12–24 hours. After cooling, check the seals: the lids should be concave and should not flex up and down when pressed in the center. Wipe jars, label, and store.