I make blackberry jam every year and last year I added a jar of it to the secondary of a wheat. It was great, and of course the jam had a ton of granulated sugar and added pectin in it but it didnt affect a wheat. Remember that blackberries have a lot of natural pectins in them so if you boil them you will definitely not get a very clear beer. For a wit thats great but for anything else you might want to use campden tablets to sterilize. Also, make sure you use a hop bag to add the berries, there are a ton of seeds in them and wild berries are a bit more tart than store-bought blackberries. For me, I love the tartness of natural berries so thats no problem for me but you might not want that tartness in your beer. Also, I have found that if you add a little OJ or Lemon Juice to the crushed berries and leave them in the fridge overnight, you get more of a full and rounded flavor. The lower pH helps bring out some great flavors from the berries that everyone just loves in my jam and everyone loved how well rounded my blackberry wit flavor was.
Jam Recipe:
1cup of sugar per cup of blackberries
1-2 tbsp of table sugar
1/4 Cup of Orange juice