Originally Posted by ericd
Okay, I haven't tried this, and I'm still at the malt-extract stage of my homebrewing development, but what I'd to is make a puree of the fresh blackberries with some of the green beer after complete fermentation and use that to prime your bottles. I tried making a cherry stout once and I had the same problem with the flavor fermenting out. I know you're making a hefe, but still...
Assume fructose = dextrose for yeast metabolism. You'd need about 630g of dextrose for a 10 gal batch of beer to reach 5 vols. carbonation. Blackberries are 8% sugar by weight, which I assume is mainly fructose. To get that much sugar from the blackberries, you'll need 630/0.08 = 7.88 kilos of blackberries. I'm not sure if 8 quarts is >= 7.88 kg, but you get the idea.
Its really tough to say how to keep the "fruit" flavor in. Yeast eats a lot of things and leaves very little. I have made a raspberry porter before and the specialty (dark malts in extract) malts overpowered the fruit. The truth is I thought I would get more taste out of a lighter hefe but I couldn't notice that much difference. Once again, it was good, but not fantastic. I used 10 pounds of blackberries in my 10 gallon brew. it was 1.050 and finished a dry 1.010. Good luck!