Step 1: Wash the blackberries gently and place them in a large, heavy-bottomed pot or Dutch oven. Add the lemon juice.
Step 2: Over medium heat, bring the berries to a gentle simmer, mashing them slightly with a potato masher as they soften. Continue to simmer for 5-10 minutes, stirring occasionally, until the berries have released their juices.
Step 3: Add the granulated sugar to the pot. Stir constantly until the sugar is completely dissolved. Increase the heat to medium-high.
Step 4: Bring the mixture to a rolling boil that cannot be stirred down. Continue boiling, stirring frequently to prevent sticking, until the jam reaches 220°F (104°C) on a candy thermometer. This typically takes 15-25 minutes. Alternatively, you can use the plate test: place a small spoonful of hot jam on a cold plate, let it cool for a minute, then push it with your finger. If it wrinkles, it's ready.
Step 5: Carefully ladle the hot blackberry jam into your pre-sterilized hot jars, leaving about 1/4 inch headspace. Remove any air bubbles with a non-metallic spatula or knife.
Step 6: Wipe the rims of the jars clean with a damp cloth. Center the hot lids on the jars and screw on the bands until finger-tight. This is also where good glass food storage containers come in handy if you're not planning to can them.
Step 7: Process the jars in a boiling water canner for 10 minutes (adjusting for altitude if necessary). After 10 minutes, turn off the heat, remove the canner lid, and let the jars sit in the hot water for 5 minutes before carefully removing them to a wire rack to cool completely for 12-24 hours. You should hear popping sounds as the lids seal. Store sealed jars in a cool, dark place for up to a year. Refrigerate any unsealed jars or opened jars.