Well, don't beat yourself up too much. I know recipes have you using water, and it's so that you get a specific amount of wine--like 5 gallons or whatever--so that measurement of the chemical additions is easier for you. But recipes don't make great wines. Blackberry can be hard to work with--most people use malic metabolizing yeasts, and a PH meter in order to bring the acidity in line. Without that, you can make a real harsh wine that you won't enjoy too much. At least your water additions will help control the low PH somewhat.
We let our blackberry age from 9 months to a year. When you reach that point, and if the wine has flaws that you don't like, let us know because there are measures you can take to make it somewhat better.
Whatever you do, don't toss it out--give it a chance. I just hope your ABV won't be terribly high.