Wash jars, lids, and bands in hot, soapy water and rinse well. Place jars in a canner or deep stockpot, cover with water, and bring to a gentle simmer; keep them hot but not boiling hard. Place the lids in warm (not boiling) water according to manufacturer instructions while you make the jelly.

Rinse the blackberries gently in cool water and drain. Add berries to a large pot, lightly crushing them with a potato masher or the back of a ladle. Add just enough water to barely cover the bottom of the pot (about 1/2 cup) to prevent scorching. Bring to a simmer over medium heat, stirring occasionally and mashing now and then, and cook 10–15 minutes until very soft and juicy.

Line a fine mesh strainer with 2–3 layers of cheesecloth or set up a jelly bag over a large bowl. Pour the cooked berries and juice into the strainer or bag. Let the juice drip through for 30–60 minutes. For the clearest jelly, avoid pressing hard; for a slightly cloudier but higher-yield jelly, you may gently press. Measure 4 cups of juice, adding a small splash of water or apple juice only if needed to reach exactly 4 cups.

Pour the 4 cups of blackberry juice into a large, wide pot. Stir in the powdered fruit pectin and bottled lemon juice, whisking well to dissolve any clumps. Add the butter, if using, to help reduce foam. Use a pot with plenty of room, as the jelly will bubble up during boiling.

Over medium-high heat, bring the blackberry juice mixture to a full rolling boil that does not stop bubbling when stirred. Stir frequently to prevent sticking and scorching on the bottom of the pot.

Once at a full rolling boil, add all of the sugar at once. Stir constantly until completely dissolved, then return the mixture to a full rolling boil. Boil hard for about 1 minute, stirring constantly. If using a thermometer, the mixture should reach around 220°F (at sea level). Turn off the heat. Skim off any foam from the surface with a spoon for a clearer jelly.

For a wrinkle test, keep a small plate in the freezer while the jelly cooks. After the 1-minute hard boil, drop a spoonful of hot jelly onto the cold plate and let sit 30 seconds. Push the edge with your finger; if it wrinkles slightly and does not flow back quickly, it has reached the jelly stage. If it is still very runny, return the pot to the heat and boil another 30–60 seconds, taking care not to overcook.

Using a jar lifter, remove hot jars from the canner and place them on a towel-lined counter. Using a ladle and canning funnel, fill each jar with hot blackberry jelly, leaving 1/4-inch headspace at the top. Run a clean, damp spatula or plastic knife around the inside of each jar to release air bubbles. Wipe jar rims with a clean damp cloth. Place lids on the jars and screw on bands until fingertip-tight—snug but not overly tight.

Place the filled jars back into the canner, ensuring they are covered by 1–2 inches of water. Bring the water to a steady boil, then process (boil) for 10 minutes for half-pint jars at sea level, adjusting processing time as recommended for higher altitudes. When the time is up, turn off the heat, remove the canner lid, and let the jars sit in the hot water for 5 minutes.

Lift jars out carefully and place them on a towel, leaving space between jars. Do not tighten the bands or press on the lids. Let cool, undisturbed, for 12–24 hours. Check seals: lids should be flat or slightly concave and not flex when pressed. Any unsealed jars should be refrigerated and used first. Properly sealed jars can be labeled and stored in a cool, dark place.