How to Make It
Place a small plate in the freezer. In a nonreactive medium saucepan, combine the blueberries with 1/2 cup of water and bring to a boil over moderately high heat. Stir in the sugar and lemon juice and cook over moderately low heat, skimming off any foam as necessary, until the liquid has thickened and most of the blueberries have burst, about 40 minutes.
Spoon 1 tablespoon of the jam onto the chilled plate and refrigerate until it comes to room temperature, about 3 minutes; the jam is ready when it thickens like jelly and a spoon leaves a trail when it’s pulled through. If necessary, continue simmering and testing. Spoon the jam into a 1-pint jar, leaving 1/4 inch of space at the top. Seal the jar and let cool to room temperature.