I have been making this vinegar for years, and it can be used in such a variety of ways, that I really cannot imagine my pantry being without it.
I use it as a salad dressing, with chicken, turkey, and fish dishes.
You do not have to use blackberries, as raspberries work well in this too. I imagine that strawberries would be a lovely substitute as well, but I haven't tried them as I'm allergic to them.
Place berries into a crock, or other non-reactive and non-metallic container. Not in plastic either! Ewww!
Cover with the vinegar, put a lid or plastic wrap over the container and let stand 3 days, stirring once each day.
At the end of this, pour all into a cheesecloth-lined strainer that is set over another container. Invert a plate over the top, weight it down with something heavy like a large can of tomatoes and let drain all day, or over night.
Measure juice and add 1 lb sugar per pint of juice.
Bring to the boil, then reduce to the simmer for 5 minutes.