Combine blackberries and sugar in a bowl. Lightly crush the blackberries to release their juices (I used a potato masher, but a fork would suffice). Cover and let sit 8 hours or overnight. Shake the bowl every once in awhile to make sure the sugar is incorporating into the juice. Half way through, stir the mixture and re-cover.
Next, strain the mixture through a fine strainer into an airtight container. Add the vinegar and shake well (if any sugar has remained, shake hard to dissolve here). Store the container in the refrigerator for 6 days.
After 6 days give it a taste. Usually, by day 6 the sharpness of the vinegar has started to pull back and let the sweetness from the sugar and fruit stand out more. Keep in mind, this is a vinegar base: it will ALWAYS taste like vinegar. The vinegar will mellow more as it sits but its zing is what is wanted in a shrub.
For this shrub, there is a nice sweet and sour balance from the ingredients. The blackberries produce a tartness that is heightened from the vinegar while the sugar cuts through to keep your mouth from puckering. Strong nose of vinegar with subtle berry.