Fill a large pot half full of water. Bring to a rolling boil, add blueberries. When the water returns to the boil, start timing 1 minute. Then drain the berries out (discard the blanching water), return them to the pot, and cover the pot to keep the berries warm.
In a separate large pot, add the fresh water, sugar, lemon juice and zest (if using), and food colouring (if using). Stir well. Sprinkle the Clearjel over the surface; whisk well to mix all in.
Put on stove burner, stirring very constantly. As soon as it starts to heat up, the Clearjel will thicken very suddenly on you. Keep whisking then remove from heat.
With a spoon, carefully fold in the drained warm blueberries.
Ladle into half-litre (1 US pint / 16 oz) jars or 1 litre (1 US quart / 32 oz) jars.
Leave 3 cm (1 1/4 inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar for 30 minutes; increase time as needed for your altitude.