Place a small plate in the freezer to use for testing the jam.
Wash and drain 1 cup blueberries in a colander.
Add blueberries, ⅓ cup sugar, 1 ½ teaspoons lemon juice, and 1/16 teaspoon kosher salt to a small saucepan. Bring to a boil and allow the blueberries to simmer and thicken for about 10 minutes. Gently smash the blueberries with the back of your spoon or a potato masher to encourage them to break down.
Take your frozen plate out of the freezer. Spread a small amount of jam across the frozen plate. The frozen plate will instantly cool the jam down so that you can determine how thick it is. Tilt the plate. If the jam slides across the plate quickly it needs to boil and thicken for longer. If the jam slides slowly it is done.
Store the blueberry jam in an airtight container in the refrigerator for up to one month.
Notes
Don’t overcook the fruit. Overcooked jam will be tough and rubbery. The mixture will seem too thin but will thicken up as it cools.Boiling sugar can easily cause burns. Use caution and be careful to not splatter the mixture.The homemade blueberry jam will thicken as it cools and the natural pectin present in blueberries sets.If you want to add fresh herbs to your jam, add them at the end of the cooking process to preserve their bright flavor.Storage: Refrigerated jam should be good to eat for at least a month. Always use clean utensils to retrieve jam from the jar to avoid contamination.Signs of spoilage include mold or yeast growth, change in color, or any off odors like the smell of yeast, alcohol, or fermentation.