Mash the blackberries a layer at a time in something like a pie plate using something like a potato masher.
Press through sieve or food mill to remove seeds, if desired (see discussion in Notes below.)
Measure 1 litre / 4 cups of juice and put juice in large stainless steel pot.
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, ladle out about 4 tablespoons of juice and put in a small bowl. Add the pectin powder all at once, and stir it into juice. Whichever combination you made up, set it briefly aside.
Add lemon juice to the pot.
Add the calcium water to the pot.
Bring the pot to a boil over high heat.
Add the pectin mixture a little at a time, stirring constantly.
Let boil vigorously for another 2 minutes, stirring constantly.
Remove from heat.
Ladle into 125 ml (4 oz) or 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 either size jars for 10 minutes; increase time as needed for your altitude.
Sweetener suggestions: 1 cup (200 g) sugar or 1 cup (25 g) granulated Splenda OR ½ cup up to 1 cup (125 to 250 ml) honey or 2 teaspoons liquid stevia. For sugar, recipe says you can use ¾ cup up to 2 cups sugar. We used 1 cup.