Place a metal spoon or glass plate in the freezer.

Pour the blackberries in a blender or food processor. Pulse the berries several times until the berries have broken down into smaller pieces. If you don’t have a blender or food processor you can mash the berries by hand. A potato masher works well for this.

Strain the berries through a food mill on the largest blade to remove the skins. Repeat the process using the next smaller blades to remove the seeds. You can also strain the berries through a mesh sieve. If you don’t mind having seeds in your jam, you can skip this step.

Transfer the berries to a medium saucepan.

Add the lemon juice.

Bring the berries to a rolling boil over medium high heat, then add the sugar. Continue to boil for 1-2 minutes, stirring constantly.

Test the jam for gelling by dropping a few drops of the hot jam onto the plate or spoon you put in the freezer earlier. If the jam gels to your desired consistency, remove the jam from the heat. The jam will continue to thicken as it cools.

If the jam is still too runny, continue to boil in one minute increments, using the cold spoon or plate test in between.

Transfer the jam into clean and sterilized jars. Store in the refrigerator or process using the water bath canning method.