Optional: 1 teaspoon powdered pectin tossed with 1 tablespoon sugar
Instructions
In a large saucepan or Dutch oven, combine blackberries and sugar. Let stand for 15 minutes.
Use a potato masher to mash the berries in the pot, releasing the juices.
Bring the mixture to a boil over medium heat. Once the mixture boils, add the lemon juice; cook, stirring frequently, until the mixture thickens and coats the back of a spoon, about 15-20 minutes.
For a thicker jam, whisk together pectin and 1 tablespoon of sugar. Sprinkle the pectin mixture over the jam and stir to combine. Cook for exactly 1 more minute, then remove from the heat.
Divide the jam between 2 sterilized 8-ounce glass jars, leaving ¼-inch of headspace at the top of each jar. Wipe the rims clean with a damp cloth, then secure the lids and bands.
Process in a water bath for 10 minutes. Carefully remove the jars to a towel on the counter. Let stand, undisturbed, for 12 hours at room temperature. Check the seals after 12 hours by pressing the center of the lids. If the lid gives or the center button remains popped up, store in the refrigerator for up to 1 month. Properly sealed jars will keep in a cool, dark place for up to 8 months. Refrigerate after opening.
Notes
Weigh the berries -- don't measure by volume. In canning, the ratio of sugar to acid is important to create a safe product that properly sets. To account for differences in produce sizes, I always recommend weighing your blackberries on a kitchen scale to make sure that you have exactly 1 pound of fresh fruit.
Leave ¼-inch of headspace in each jar. The proper amount of headspace is important to ensure a vacuum seal. If there's too little headspace, the jam may expand and bubble out when air is being forced out from under the lid during processing.
Bottled lemon juice (rather than freshly-squeezed lemon juice) has a consistent level of acidity to ensure a safe product that sets properly.
If you don't want to bother with canning the jars in a water bath, no problem! Just transfer the jam to jars, let them cool completely at room temperature, and then store in the refrigerator for up to 3 weeks.