Wow, how do you do that? I've got tons (to many by far) wild blackberries growing around here. It's been a bumper year for them. How much blackberry do you use? What kind of yeast?
My recipe used 18 pounds of crushed blackberries, 12 pounds of sugar total, two crushed campden tablets, and I forget the kind of yeast except it is a regular wine yeast and it took two packets. This is for a five gallon recipe. I am told blackberries have a lot of natural yeast but because the sources of the starter are questionable (animal urine for example) it is best to use the campden tablets to purify. Here is the site for the recipe I used. http://www.familyherbalremedies.com/blackberry_wine_recipe.html