Combine the fresh black raspberries and water in a large heavy-bottomed pot. Heat over medium-high heat, stirring frequently and mashing the berries against the side of the pot with a wooden spoon as they soften. This should take about 10-15 minutes until the berries are completely broken down and the mixture is very soft. The goal is to release as much juice as possible from the fruit.
Line a fine-mesh strainer with cheesecloth or use a jelly bag and set it over a large bowl. Carefully pour the cooked raspberry mixture through the strainer, allowing the juice to drip into the bowl—do not press or squeeze the fruit, as this will make the jelly cloudy. Let it drain completely for at least 30 minutes. Measure out exactly 4 cups of the strained juice; if you have less, add a small amount of water to reach 4 cups.
Pour the strained black raspberry juice into a clean large pot and add the granulated sugar and fresh lemon juice. The lemon juice adds pectin to help the jelly set properly and brightens the flavor. Stir well to combine, making sure all the sugar is moistened.
Bring the mixture to a rolling boil over high heat, stirring occasionally to ensure the sugar is fully dissolved. Once boiling, I like to add a small dab of butter at this stage—it significantly reduces the amount of foam that forms on top, which means less skimming later. Continue boiling for 8-10 minutes, checking the temperature with a candy thermometer. The jelly is ready when it reaches 220°F (or 8°F above the boiling point of water at your altitude). You can also test by dropping a small spoonful onto a cold plate; if it wrinkles when you push it with your finger, it's ready.
Remove the pot from heat immediately when the gel point is reached. Using a skimmer or slotted spoon, gently skim off any foam that has formed on the surface and discard it. Let the jelly rest for 1-2 minutes so any remaining air bubbles rise to the surface.
While the jelly is still hot, carefully ladle it into clean, hot sterilized jars, leaving exactly 1/4 inch of headspace at the top. This space is crucial for proper sealing. Wipe the rim of each jar with a clean damp cloth to remove any spilled jelly, then place sterilized lids and screw bands on firmly but not too tight—they should be finger-tight. Place the filled jars into a boiling water bath (water should cover jars by 1-2 inches) and process for 10 minutes. After processing, remove the jars with a jar lifter and let them sit undisturbed in the hot water for 5 minutes before transferring to a cool, draft-free spot. Allow the jars to cool completely for 12-24 hours without moving them—you should hear a satisfying 'pop' as each jar seals.
