Mix the blackberries with the sugar in a heavy-based saucepan, add the lemon juice, scrape the seeds from the vanilla pod and add the seeds and the pod.
Bring slowly to the boil and simmer for 5 minutes. Mix the sugar and pectin and sprinkle it in a thin stream into the boiling blackberries, stirring continuously.
Bring to the boil again, then pour the jam into sterilised jars. Leave them to cool, then refrigerate.
The blackberry jam will keep unopened in the fridge for approx. 4–6 months and for approx. 2–3 weeks in the fridge after opening.
