Put the blueberries into the pot with the mashed rhubarb.
Add the calcium water to the blueberries and rhubarb. If using sugar or Splenda. add the lemon juice now. Otherwise, do not add it yet.
If using sugar or Splenda. put them in a bowl, add the pectin powder, and mix. If using a liquid sweetener such as honey or liquid stevia, put instead the lemon juice in a bowl, add the pectin powder all at once, and stir it into the lemon juice.
Put the fruit pot back on the burner and bring to the boil.
When the pot has come to a full boil, stir in the pectin mixture a dollop at a time. If using a liquid sweetener such as honey or liquid stevia, add that to the pot now too.
Boil for another minute.
Remove from heat.
Ladle into quarter-litre (½ US pint / 8 oz) 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 for 10 minutes; increase time as needed for your altitude.