Wash your canning jars and lids in soapy, hot water (no need to sterilize, as we will process the jars for 10 minutes). Keep jars warm until ready to be filled with jam (I just keep them submerged in warm water).

Place blueberries in large pot, heat over medium heat, and mash with potato masher until you've mashed most of the blueberries. It will look like a thick sauce.

Stir in lemon juice and water. Gradually stir in pectin (1 tbsp at a time). Increase heat to high, and while (slowly) constantly stirring bring mixture to a rolling boil (that can't be stirred down). Watch out that mixture doesn't boil over.

Add granular erythritol and bring to another rolling boil while constantly stirring. Keep the hard boil going for 1 minutes. Then turn off heat.

You may see some foam on top. Skim foam with a spoon. I love eating the foam and simply put it in the fridge. If you don't like the foam, you can simply add a bit of butter (¼ tsp) after you add the pectin. This will prevent the foam.

Fill hot jam into warm jars, one at a time. Place jar funnel into jar and with a ladle pour the jam in. Leave a headspace of ¼ inch. Wipe the rim with a moist paper towel.

Place lid on jar. Then screw on band, but just finger-tight. This will allow steam to escape. Place jars into canner (with the rack sitting at the bottom). Fill canner with enough water to cover jars with 2 inches of water. Bring to a gentle boil and boil for 10 minutes. (Adjust for altitude, see notes.)

With a jar lifter, remove jars from canner (repeat processing with uncanned jars, making sure to add more water to canner). Let jars cool down completely (about 12 hours). During this time, don't touch lids. You will hear popping sounds as the lids seal.

After the 12 hours, check the seal by pressing down on the lid center. The lid should not flex up or down. If it does, it didnt' seal correctly. In that case, you can either try processing again with a new lid or place that jar in fridge an eat within 2 weeks.

Unscrew bands from jars (leave the lids on) and dry bands with paper towel in case they are wet (this will prevent them from rusting). Screw the band back on and store in cool, dark place for about 1 year.