Fill a jar about 3/4 full of blackberries.
Pour vinegar over the berries, almost to the top of the jar.
Cover the jar with a plastic lid, or place a layer of clingfilm between a metal lid and the vinegar to avoid corrosion.
Place in a dark cupboard for 7 – 10 days.
Strain through a coffee filter or cheesecloth.
Add the liquid to a saucepan with 450g of sugar per 700ml of liquid. Bring to the boil, then boil for 8-10 minutes.
Transfer to steralised bottles.
Store in your fridge and use within 6 months.