Set seven canning bands (some call them rings) aside, near where you will be filling the jars.
Place the seven glass jars in the water bath canner. The water should cover the jars by at least an inch. Turn the heat to medium-high.
Place seven jar lids in the small saucepan with enough water to cover the lids. Cover the pan and heat the water on low.
Measure out 6 cups of frozen or fresh blueberries. Put the berries into a colander and rinse. Let drain.
While the blueberries are draining, measure out the 6 cups of sugar into a bowl.
Grate the lemon zest into a small bowl.
Cut the lemons in half and squeeze the juice into a measuring cup with a small strainer on top. You should get about 1/2 cup of juice.
Pour the drained blueberries into the large stockpot. Crush the blueberries using a round potato masher.
Mash the blueberries until you have a bit of blueberry juice flowing. You don’t need to completely pulverize the berries. It’s okay to have some whole berries left in the pot.
Add the sugar, zest, and lemon juice to the pot. Cook and stir the mixture over medium-high heat. Bring to a boil.
Boil and stir the hot mixture for 6 minutes. Remove from the heat.
Stir in the lemon jello. Continue to stir for 2 minutes until the jello is dissolved.
Using tongs, if you have them, lift hot jars out of hot water.
Bring the pot of hot jam over and place it next to the jars. Ladle hot jam into jars, leaving 1/4 inch headspace.
Once all jars are filled, wipe jar rims. Place lids on jars and place bands on jars. Adjust until they are fingertip tight.
Place jars (using a jar-lifter, if you have one) in the hot water bath in the canner. Water should cover the jars by at least one inch.
Bring the water to a boil and process the jars for 10 minutes (15 minutes for altitudes above 1,000 feet).
When processing is done, remove the jars (again using a jar-lifter, if you have one) and place on a towel to absorb the drips and to cool.
If all has gone as it should, you should hear the jars seal as they cool. When they are cool, press the center of each lid. They should not flex.
Label and store the jars in a cool, dry place. The quality is best if used within 12 months. Refrigerate any jars once opened.