First sterilize the jars in the steamer for 20 minutes at 100°.
In the meantime, puree the blueberries with the preserving sugar and the lemon juice and let them steep for 20 minutes.
Then pour the blueberry-jelly sugar mixture into the jars, seal well and steam for 1 hour at 100 degrees.
Then screw the lid on tightly again, shake and let the blueberry jam cool down from the steamer.