In a medium saucepan, combine the washed blueberries and lemon juice. Use a wooden spoon to gently mash the blueberries to release their juices. This helps to create a smoother jam.

If you prefer a chunkier texture, you can skip mashing or use a potato masher for a few seconds.

Add the granulated sugar and lemon zest to the saucepan with the blueberries. Stir well to combine all the ingredients.

Place the saucepan over medium heat and bring the mixture to a boil. Stir frequently to prevent sticking and ensure even cooking.

Once boiling, reduce the heat to medium-low and let it simmer for about 20-25 minutes. The mixture should thicken as it cooks. To test the consistency, place a small spoonful on a cold plate and let it sit for a minute. Run your finger through it; if it holds its shape, it's ready.

If you prefer a thicker jam, add the pectin according to the package instructions, usually during the last 5 minutes of cooking. Stir continuously to incorporate.

Once the jam has reached the desired consistency, remove the saucepan from heat. Let it cool for about 5 minutes.

Using a ladle, carefully pour the hot jam into sterilized jars, leaving about 1/4 inch of headspace at the top. Seal the jars with lids.

If canning, process the jars in a boiling water bath for 10 minutes to ensure they are sealed properly.

Allow the jars to cool completely at room temperature. Once cooled, store the jam in the refrigerator for up to 3 weeks or in a cool, dark place for up to a year if canned.