We freeze red/black currants spread out in a layer on baking sheets then shake them out into plastic bags so they take up less room and you can use them in whatever quantities you want - you could do the same with blackberries. You don't have to be too precious about it, just drop them on and give them a shake so they cover the tray- some will stick together but not too many. They always seem so good in the winter when there's little good fruit around