Instructions:
- Sterilize jars and lids according to standard canning practices. Place a small plate in the freezer to chill for testing the set later.
- In the cooking pot, combine blueberries, sugar, and lemon juice. Let stand for 15 minutes to allow the sugar to start drawing out the fruit juices (maceration).
- Place the pot over medium-high heat. Stir constantly until the sugar is completely dissolved and no graininess remains when rubbed between your fingers.
- Bring the mixture to a vigorous, rolling boil that cannot be stirred down.
- If using butter, add it now. Use a spoon to carefully skim off any pink or grey foam that rises to the surface to ensure a clear jam.
- Continue boiling, stirring occasionally to prevent scorching. Insert a canning thermometer and cook until the jam reaches 220°F (104.5°C).
- Remove the pot from the heat. Test the set: Place a teaspoon of jam onto the chilled plate, return it to the freezer for 1 minute, then gently push the jam with your finger. If the surface wrinkles, it has set. If not, return to a boil for 5 more minutes and test again.
- Once the set is confirmed, let the jam cool in the pot for 5 minutes. Ladle the hot blueberry jam into the sterilized jars, ensuring 1/4 inch (0.5 cm) headspace.
- Wipe the jar rims clean, affix the lids, and process in a boiling water bath for 10 minutes for a shelf-stable seal, or simply refrigerate immediately.