I've used blackberries before in porter, and had really great results with them added to the secondary, that way you get more of the berry flavor. As far as treatment of the berries, I kept them whole and steamed them in a vegetable steaming basket for ten minutes prior to adding them gently crushed to the secondary, them racked the porter on top of them. Got a secondary fermentation that lasted a week, allowed two more weeks for clearing, and racked, primed, and bottled after that. Turned out as one of the nicest beers I've made to date. I don't know how much fruit you've got, but I used 10oz per gallon and it was perfect. As far as bits of berry in my beer, they are virtually non-existent, I credit this to my two week wait after the secondary fermentation finished.