I brewed a Blackberry hefeweizen last summer and used 1 pound per gallon of frozen and thawed blackberries. I used WY3638, which tends to have a bit more vanilla/cinnamon/cardamom than clove on the phenolic side compares to 3068, but is otherwise a fairly typical hefe strain. Honestly, if I were to brew another blackberry beer I would go with a more neutral yeast. I wasn't a big fan of the hefe yeast in combo with the fruit.
I also find that blackberries tend to be a bit more astingent/tannic than I like in a fruit beer. I also didn't get as much of the fresh berry flavor I was looking for. If I were to try this again, I'd probably either use 1lb of blackberries + 1lb of raspberries per gallon to get more "berriness", or even just use a combo of fresh raspberries plus a bit of blackberry flavor extract just to kick up the flavor a bit.