Prep jars and canner. Wash jars, rings, and lids in hot soapy water. Submerge the jars in a water bath canner, cover, and bring to a simmer over medium heat. Place the lids and rings in a small saucepan of water over medium-low heat.
Blanch the blueberries. Bring a large pot of water to a rolling boil. Add the blueberries and blanch for 1 minute. Drain well in a large strainer and set aside. Reserve 2 cups of the blanching water if using for natural color.
Mix sugar and ClearJel. In a large pot, whisk the sugar and ClearJel together until evenly combined. (If using cinnamon, whisk it in now.)
Add cold water. Pour in 2 cups of cold or room temperature water. Do not use hot water, it will cause lumps. Stir in food coloring now if using (12 drops blue, 3 drops red).
Cook until thick. Bring to a boil over medium-high heat, stirring frequently. Once bubbling, reduce heat to low for a gentle boil and stir constantly until the mixture thickens. This happens quickly.
Add lemon. Stir in the lemon zest and lemon juice. Cook 1 minute more, stirring constantly.
Fold in blueberries. Remove the pot from the heat and gently fold in the blanched blueberries.
Fill jars. Ladle the hot filling into hot jars, leaving 1 ½ inches of headspace. Less headspace will cause siphoning.
Debubble and seal. Run a chopstick or debubbler around the inside of each jar to release air pockets. Wipe rims with a damp clean cloth, place lids, and screw rings on fingertip-tight.
Process. Lower jars into the canner, submerged with at least 2 inches of water above the lids. Bring to a boil, then process for 30 minutes (adjust for altitude).
Rest and cool. Turn off the heat, remove the canner lid, and let the jars rest in the water for 5 minutes. Transfer to a heat-safe rack and cool 12 to 24 hours.
Check seals and store. After 24 hours, press the center of each lid to confirm a seal. Remove rings and store sealed jars in a cool, dark place for up to 18 months.