Prep berries. If you are starting with frozen blackberries, allow them to thaw in the fridge overnight. The next day place the blackberries in a colander over a bowl and allow the blackberries to drain, reserving the juice of the blackberries.
Prep canning supplies. Wash your jars with hot, soapy water. Prepare your water bath canner by filling it with water and allowing the jars to be fully submerged in water. Place over medium heat, cover, and simmer. Place the lids and rings in a pot of water and simmer on low.
Combine sugar and clear jel. In a large bowl combine the sugar and clear jel together. Set aside.
Mix with liquid. In a large pot add the water or reserved berry liquid. (If when thawing your berries you do not get enough liquid, add water until it equals 4 ⅓ cups liquid) Add the sugar and clear jel mixture to the large pot stir to fully combine.
Heat until thickened. Set to medium heat, stirring constantly and allow the mixture to thicken and start to bubble. The mixture will become thick and if you are not stirring constantly it will stick to the bottom of the pot.
Add lemon juice and boil. Add lemon juice and stir to fully combine. Allow the mixture to come to a boil and cook for 1 minute.
Add berries. Remove from the heat and fold in the blackberries.
Fill jars. Ladle the blackberry pie filling into the prepared jars. You will want to leave 1 ¼ inch head space to prevent oozing. Using a chopstick, remove any bubbles. A wooden or plastic chopstick works better than a standard debubbler to prevent breaking up the blackberries.
Wipe and add lids and rings. Wipe jars using a moist paper towel (water or white vinegar will work) making sure that the rims are fully clean. Add lids and rings.
Process. Process in your water bath canner for 30 minutes, adjusting for altitude. Once done, turn off the heat, remove the lid of the canner and allow the jars to rest inside the canner for 10 minutes.
Cool and check seals. Remove from the canner and allow them to cool. Let them sit undisturbed for 12-24 hours. Check the seal. Once a seal has been confirmed, remove the ring and store it in a cool, dark place for up to 18 months.