In a large saucepan combine the blackberries, sugar, lemon juice and salt. Bring to a rolling boil and cook stirring frequently for 15 to 20 minutes or until the temperature of the mixture has reached 105 degrees C.
Transfer the mixture to hot sterile jars leaving 1cm headspace then seal. Process jars for 10 minutes in a water bath. Refrigerate jam once the seal is broken.