Slightly under ripe fruit is better of jam, I think blackberries (here anyway) are not high in natural pectin, so I use 'jam sugar' when making it, and a little lemon juice, no water. I add the sugar to the blackberries and leave to 'mix' overnight, and make the jam the following day. There will be plenty of juice/liquid in the pan. Heat slowly until the sugar dissolves, the bring to a boil until setting point is reached.
I also like to soak blackberries in whiskey, leave to soak for about 8 weeks, drain and store to mature. Delicious.