Bring the water to a boil and then add the petals. Turn off the heat and cover the pot with plastic wrap. Let steep for 4 to 5 hours. Strain the petals out and remeasure the water; you should have 120g/4.23oz. Let it cool completely.
Meanwhile, line 10 PVC tubes measuring 15cm/6in long by 1.25cm/.5in diameter with acetate plastic sheets. The acetate should protrude from both ends of the PVC tube because they will be easier to pull out of the PVC than if they were the same size as the tube.
Place them on a sheet pan in the freezer to get cold.
Combine the rose petal water with the blackberry purée and the sorbet syrup using a whisk.
Churn the sorbet base and pour it into a piping bag. Pipe the sorbet into the frozen PVC tubes. Return the tubes to the freezer to harden, about 2 hours.
Once they have hardened, remove the frozen sorbet, acetate and all, from the PVC tubes. Reserve the acetate-wrapped tubes frozen. If you want to get ahead, trim the ends of the sorbet using a sharp, thin knife so that the tube measures exactly 15cm/6in long. Keep the tubes covered while frozen. They may be kept in the freezer for up to 4 days. After that, melt them down and re-churn them. Sorbets can be rechurned more often than ice creams.