This is fermented, cleared wine already? Adding blackberries will add sugar, which will restart a ferment, unless you add sorbate and are properly sulfited. What is your goal for the blackberry/petit sirah? A dry wine?
I would add Potassium sorbet then make a syrup from frozen blackberries and add. Amount would depend on strength of flavor you want, I'd start with a couple of cups worth.
Mike
the first 3 gallons will be and is on the dry side which is fine with me. my goal is to make the other 3 with a hint of blackberry flavor, and will sweetening it a tad.