Sort blackberries. Mix the blackberries, jam sugar and lemon juice in a large, tall pot. Bring to the boil while stirring over high heat and let it boil fizzily for at least 3 minutes.
If necessary, skim and pour hot into prepared, sterilized glasses and close them tightly. Then allow to cool well. Blackberry jam keeps for at least 6 months if stored in a cool and dark place.