Mmmm, blackberry wine. Here is a recipe for one gallon. The handwritten notes are what I did differently and quantities of ingredients. I pretty much used pure juice on this, but you can add water as per recipe. I like sweet very flavorful wines so used the same amount of sugar but more juice, which is why it started at a higher S.G.
If you failed on a previous batch, you likely had contamination. Sterile technique is very important as is reducing the natural yeasts and bacteria in your fruit until the yeast you add takes over. I used Premiere Cuvee yeast, if I recall, I've always been happy with it.