Mash the blueberries a layer at a time in something like a pie plate using something like a potato masher.
Add to a large pot.
Add fruit juice, stir.
Sprinkle the pectin over, stir until the pectin is dissolved.
Put on burner, add the sweetener you are using, and bring to a boil, stirring constantly.
Then, boil hard for 3 minutes.
Immediately turn burner off and remove pot from heat.
Skim off any foam, if there is any.
Ladle into hot quarter-litre (½ US pint) jars.
Leave 1 cm (¼ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process jars for 10 minutes; increase time as needed for your altitude.
Notes
1 kg (7 cups) of whole blueberries, after mashing, will equal about 1 litre (4 cups) of mashed. A tidge over or under is fine. But if you measured your berries by weight, you won't be too far off that.
If using frozen blueberries, zap from frozen in microwave for 4 to 5 minutes, then mash, retaining any juice that might have come off while thawing.
Don't crush or mash the berries in a machine, it will damage the pectin.
For the fruit juice, you can use cranberry, apple, grape, etc.
In 2015, the recipe called for 1 cup (200 g / 8 oz.) of sugar. Sometime between then and 2022, they boasted the amount to 1 ½ cups of sugar. As no sweetener at all is needed for either the safety or success of this recipe, feel free to use less than the new higher amount if you wish -- it depends on the tastes of your crowd and how sweet / tart that particular batch of blueberries was.
How much Splenda or stevia you need will again depends on the tastes of your crowd and the blueberries in question.
Ball / Bernardin No Sugar Needed is the same; it's made in the same plant and just labelled with different names on the packaging. If you are using the Ball No Sugar Needed pectin from a canister, you'll want 3 level tablespoons.
Boil hard means it should bubble for the whole 3 minutes.
You could also use the 125 ml (½ cup) smaller sampler size jars. But this recipe hasn't been tested for larger ½ litre (1 US pint) jars.