Rinse 4 cups blackberries and remove stems
Mash the berries in a large pot
Add 2 cups sugar and 2 tablespoons lemon juice
Stir over medium heat until the sugar dissolves
Bring the mixture to a boil
Cook, stirring often, until thickened, about 15 to 25 minutes
Test the jam by placing a small spoonful on a cold plate
Remove from heat when it reaches desired thickness
Skim off any foam from the surface
Pour the hot jam into clean jars
Seal the jars and let them cool
Refrigerate and use within a few weeks, or process for longer storage