Prepare a canning pot and enough jars to hold between 4 and 5 half pints (yields vary!).

Combine the rhubarb, blackberries, 1 cup sugar, and the calcium water in a low, wide, non-reactive pot. Stir vigorously to combine, with the intention of breaking up the berries. Let the fruit sit with the sugar for a few minutes, so that the sugar dissolves and you get a little syrupy liquid in the bottom of the pot.

Set the pot over high heat and bring to a boil. Reduce the heat to medium high and cook until the berries break down, the rhubarb is on the verge of dissolving, and the liquid thickens.

Whisk the pectin powder into the reserved sugar. Add the pectin-spiked sugar to the cooking fruit in two or three batches, stirring to integrate between each addition.

Cook for an additional minute or two, until you see visible signs of thickening.

Remove the jam from the heat and funnel it into prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.