Thoroughly wash the blueberries and thyme, ensuring you drain excess water from the blueberries before starting the jam-making process.
In a suitable-sized pot, combine the blueberries and sugar. Gently remove the leaves from the thyme sprigs and add them to the blueberries. Using a potato masher, lightly mash the blueberries. Allow the mixture to rest for about 1 ½ hours, or even overnight, to allow the flavors to infuse. Cover the pot with a lid during this resting period.
Stir in the powdered pectin and bring the mixture to a boil over medium-low heat. Reduce the heat to a simmer and skim off any foam that forms, if desired.
Let the jam simmer for 10 minutes. Turn off the heat and set the jam aside for at least two hours. It's preferable to do this in the evening and allow it to sit overnight. Repeat this step three times. Alternatively, you can simmer the jam for about 20 minutes or until it reaches your desired consistency. Avoid overcooking the jam.
(!) While the jam is cooking, begin preparing the Mason jars. It's important to have your jars ready at this point, because the next step, filling them with piping hot jam, must follow immediately.
Notes
The specified in this jam recipe quantity of ingredients should yield enough of jam to fill two 8 oz. (250 ml.) mason jars. The final yield may vary based on your preferred level of jam doneness.