You can tell a blackberry is ready to pick when it practically melts off its stem while you're pulling it off the bush. Usually at this point, blackberries have lost their sheen and are dark and perfectly black. My blackberries weren't all fully ready when I made this jam, as you can see from the picture, but I needed enough to make jam because I had enough that were crushing under the weight of their fellow berries and forming blackberry juice.
For the past couple of weeks, my fingers have been stained with blackberry juice, which has been lovely--and has made me even more excited for making blackberry jam (especially once my friend Rachael shared that she was making blackberry jam while listening to Iron & Wine one recent evening).
This is a simple blackberry jam, sweetened with agave nectar since some of my berries were under-ripe and tart.
Agave-Sweetened Blackberry Jam
1/2 gallon fresh blackberries, well washed
2-3 tablespoons agave nectar
1 tablespoon lemon juice
In a medium pot, over medium-low heat, cook the blackberries until soft, then stir in the agave nectar and lemon juice.
Continue to simmer over medium-low heat, stirring occasionally until the jam has thickened to the desired consistency.