Round about now, your blackberries are starting to get a bit dodgy, and you're either going to waste them, or turn them into jam.
Turning them into jam is a massive pain in the arse because you've got to find pectin from somewhere, and what's the point of paying for pectin to cook foraged berries?
What you can do instead, is make blackberry and elderberry jam - which can also be forraged. Elderberries have their own pectin, so you don't need more. Mix at an elderberry to blackberry ratio of around 2:1. Just cook in a tiny bit of water, until the fruit starts splitting, then add sugar of equal weight to the fruit, boil it all in, and...that's it.