If using frozen blackberries, thaw in microwave. Then add the berries and all their juice to a large bowl, toss with the 1 cup sugar, and let stand 15 minutes then mash with a potato masher.

Bring this mixture to a boil over medium heat. once it boils, add the lemon juice and cook, stirring frequently, until the mixture thickens and coats the back of a spoon, about 15-20 minutes.

During this time, clean and sterilize your two 8-oz (half-pint) jars, lifters, wide-mouthed funnel, lids, rings, and all other canning paraphernalia. Boil water for processing if you're going to process.

If using the pectin for a more certain set (recommended that we do this, especially with frozen berries), combine the pectin and 1 tbsp white sugar. Sprinkle this over the jam and stir to combine. Cook for exactly 1 more minute, then remove from heat.

Divide jam into two jars, leaving 1/4 inch of headspace at the top of each jar if you're going to process them or 1/2 inch of headspace if you're going to freeze them. Wipe the rims clean with paper towels, then secure lids and bands.

If processing: Process in water bath for 10 minutes. Carefully remove to a towel on the counter and let stand, undisturbed, for 12 hours at room temperature. After 12 hours, check the seals by pressing on the center of the lids. If the lid gives or the center b utton remains popped up, store in refrigerator for up to 1 month. Properly sealed jars will keep in a cool, dark place for up to 8 months. Refrigerate after opening.

If not processing, store in fridge or freezer. Will last in the fridge at least 3 weeks.