In a medium saucepan, over medium-high heat, stir together blueberries, sugar, lemon zest, lemon juice, and salt;
Stirring frequently, bring to a boil.
Reduce heat to medium.
Simmer, stirring occasionally while gently crushing some of the blueberries; 7 to 10 minutes, or until slightly thickened and a digital thermometer reads 220°F.
Remove from the heat and allow to cool 5 minutes in the saucepan before transferring to jars.
Leave jars unopened and let come to room temperature.
Once the jam is at room temperature, seal jars and store in the refrigerator.
The jam might take 24 to 48 hours to fully set up, but it can be eaten immediately.
