Rinse 3 pounds of fresh blackberries under cool water, discarding any soft or moldy berries.

Place the berries in a large pot and crush gently with a wooden spoon to release their juices.

Heat the crushed berries over medium-high heat and bring to a gentle boil, stirring occasionally; cook for 10 minutes.

Pour the cooked berries through a fine-mesh strainer lined with cheesecloth into a clean bowl; let the juice drip without pressing.

Return the strained juice to the pot, add granulated sugar and lemon juice, and stir until sugar dissolves.

Increase heat to high and bring to a rolling boil, stirring frequently; insert thermometer and boil until 220°F is reached.

Perform a cold plate test by chilling a small plate, dropping a spoonful of jelly, and checking for wrinkles; if it runs, boil 1 more minute and test again.

Ladle the hot jelly into warm glass jars, leaving 1/4 inch headspace; let jars cool to room temperature, then refrigerate or process in a 10-minute water bath for shelf stability.