lucky you, I wish I had that access to blackberries. I'm currently drinking an Am. Hefe that I intended to make a blackberry ale but instead went with 5 lbs of the frozen berry mix (blueberry, raspberry, blackberry). It has ended up fantastic, next time I'll go with more like 3 lbs and a touch more hops to balance the sweetness. The color on this berry brew is awesome though, literally glowing.
I like the 1 lb/gallon starting point for pure blackberries, they tend to be a bit more subdued than the others I used.