In a saucepan over high heat, stir together the blackberries and water. Bring to a boil. Mash with a potato masher or a wooden spoon and the side of the pan. Continue to boil, stirring frequently, for 15 minutes.
Remove the seeds by straining them through a fine mesh sieve or a chunk of cheesecloth. Discard the seeds and pulp and return the liquid to the saucepan.
Whisk in the remaining ingredients and bring to a boil over high heat, whisking constantly. Allow to boil for about 1 minute.
Turn the heat down to medium-low and allow to simmer for 30 to 45 minutes or until reduced by about half. The sauce should thicken as it reduces.
Pour into airtight containers such as mason jars and pop in the fridge overnight to allow the flavors to develop.
Serve cold or reheat over medium heat before serving.