Where out of interest?
I'd love to do this but wouldn't know where to start. I just don't do the countryside so I wouldn't have a clue. Where would you even go to find a blackberry?
The woods on the edge of Ditchling Road park, the old railway in Hangleton, the path leading to the downs from Horsdean rec, High Park woods at old Boat Corner - the list endless. Go to the edge of the city and find a path towards the downs and you'll do fine.