Place the blackberries and 1/2 Cup water in a small sauce pan, warm up until the blackberries are softened.
Use a fine mesh strainer over a mixing bowl, then place the softened blackberries into the strainer and use a spatula to smash them down. Do this until all the juice is into the bowl and the seeds are left in the strainer.
Combine the juice from the blackberries with the vinegar, juice from 1/2 a lemon, sugar, olive oil, and garlic salt.