I have brewed two black berry ales with Nelson Sauvin hops and I have found that if you want berry flavor, you need to add A LOT of berries. I did 1lb of blackberry per gallon and that gave me some blackberry in the background that was very pleasant. I had 5lbs of blackberries sitting for 8-10 days i believe. I wanted the Nelson to come through more so I added less blackberry the second time, about 3.5lbs for 5 gallons.
Also, adding frozen fruit has always been the best strategy for me. The cold temperature breaks down the cell walls and allows more of those nice flavor compounds to escape. So a few take-aways
1. Add LOTS
2. Add frozen
3. Add in secondary