Mash blackberries with a potato masher or a fork in a large bowl.
Stir together berries, sugar, pectin, and lemon juice in a 12-inch nonstick skillet, then boil, stirring occasionally, until slightly thickened, about 7 minutes.
Transfer jam to a large shallow bowl and chill, its surface covered with wax paper, until softly set, at least 30 minutes.