Rinse the blueberries under cold water and remove any stems or leaves.

In a large pot, combine the blueberries, sugar, and water. Bring to a boil over medium heat.

Reduce heat and let simmer for about 20 minutes, stirring occasionally, until the blueberries break down.

If desired, mix cornstarch with a little water to create a slurry and add it to the pot to thicken the syrup.

Stir in the lemon juice and remove from heat.

Sterilize mason jars and lids by boiling them in water for 10 minutes.

Using a ladle and funnel, fill the jars with the hot blueberry syrup, leaving about 1/4 inch of headspace.

Wipe the rims of the jars with a clean cloth to remove any residue. Place the lids on and screw on the bands until fingertip-tight.

Process the jars in a canner for 10 minutes, adjusting for altitude if necessary.

Remove jars and let them cool completely on a clean towel or rack.