Mix vinegar, fish sauce, sugar and cinnamon together and heat in a saucepan over medium heat just until the sugar dissolves.
Allow to cool to room temperature
Pour the liquid into a jar and add the blackberries. Swish the blackberries around gently to make sure that they are covered by the vinegar mixture. Allow to sit for at least 4 hours. Allow to sit Overnight for best result.
Video
Notes
Easy way to preserve fresh blackberries and SO tasty! See the tips and tricks section of this post!