Blackberry makes a very, very good dry wine.
4#/gallon is sufficient, I've found it doesn't matter that much, ripeness has more impact than anything else.
I'm going picking tomorrow, 2nd weekend in August is always the start of picking in the pacific NW. I could dig out my recipe, but it really is simple:
~24lb berries (assume 6ish gallons)
3 tsp (I think) pectic enzyme
tannin (follow package directions) Don't skip this or you will wonder why the wine is so flat.
water & sugar
6 campden tablets
6 tsp yeast nutrient
I usually blindly add acid blend, 6 tsp for this recipe, but you should take a reading and adjust it.
Some red wine yeast, I usually use some lalvin k1-v1116, but I don't think it matters that much.
Blackberry wine is very forgiving and you do not want to sweeten it, it is much better dry. I can always hit .990 with blackberry and I usually let it age 8 months before drinking.