Before you begin, prepare a water bath canner, jars, and lids.
Start picking over the blackberries, removing any stems, leaves or debris. Then wash and drain the fruit. (If using frozen fruit, thaw them in a colander over a bowl and reserve the juice.)
Bring a few quarts of water to a boil on the stove and blanch the blackberries for 30 seconds to 1 minute until they're heated through. Work in batches, and be gentle with the fruit so it doesn't fall apart. Remove them with a slotted spoon and keep them warm while making the clear gel mixture. (This step is important, see notes for why.)
Combine sugar and clear jel in a large pot, big enough to hold all the fruit comfortably. Add cold water or cold juice to the pot and whisk to dissolve the clear jel completely. This is important! Do not turn on the heat until the clear jel is fully dissolved into the water. See notes for why below.
Turn on the heat to medium-high and whisk continuously until the mixture begins to bubble and thicken. As soon as it bubbles, add lemon juice and cook for 1 additional minute.
Turn off the heat and fold in the drained berries.
Immediately fill the canning jars with the mixture, taking extra care to de bubble the jars as the mixture will be thick. Leave 1-inch headspace, and don't skimp here. Be sure the headspace is no less than 1 inch, but no more than 1 1/2 inch.
Cap the jars with 2 part canning lids and twist on rings to finger tight. Process in a water bath canner for 30 minutes, adjusting for altitude. Canning times are the same for pints and quarts.
Once the processing time is complete, turn off the heat and allow the jars to sit undisturbed in the canner for about 10 minutes. This reduces the risk of siphoning as you remove the jars from the canner. (See notes for why.)
Remove the jars from the canner with a jar lifter and allow them to cool on the counter for 12 to 24 hours. Check seals, and store any unsealed jars in the refrigerator for immediate use.
Properly canned and sealed jars of blackberry pie filling will maintain peak quality on the pantry shelf for about 12 months. Refrigerate after opening.