Place blackberries in a food processor bowl and process until smooth. Combine blackberry purée and vinegar in a bowl; cover and let stand at room temperature for 24 hours, stirring occasionally.
Strain mixture into a large saucepan; discard seeds. Stir in sugar; bring to a simmer, and cook for 5 minutes.
Remove from heat, and let cool.
Pour vinegar into decorative jars, and seal with airtight lids. Can be stored for up to three months.Servings: 2 cups