Wash your blueberries and pick-out/discard any impurities.
Place fruits into cooking pot, add sugar, lemon juice and lemon halves.
Heat up jam and stir occassionally. Keep over a medium heat setting.
Bring to a rolling boil, reduce heat then and cook over a slow to medium heat setting. Cook it down until jam appears shiny and a bit transluscent.
Test if the jam is ready by placing a few drops of the jam on an ice cold plate. The jam is ready if it's not running, if the opposite is the case, just keep it to cook for some more time and repeat the test. Don't cook the jam on high heat! Use candy thermometer if you need help getting the setting temperature right. See FAQs in post.
Fill your clean jam jars with the blueberry jam, close well with the lids and turn them upside down to create a vacuum. Keep them upside down until the jar has cooled a bit and then store them in a dry and cool place away from sun (lid up). Once the jar is opened, store in the fridge. Keeps well for a year and more if stored correctly.