› recipes › blackberry-jamBlackberry jam recipe | BBC Good Food Skim off any excess scum, then stir in the knob of butter, if you want – this will help to dissolve any remaining scum. Leave the jam for 15 mins before ladling into sterilised jars – this allows the fruit to settle so it doesn’t sink to the bottom. The jam will keep in a cool, dark place for at least 6 months. Refrigerate jars once opened.