In a small saucepan, combine jam through chile powder. Bring to a simmer and stir occasionally, breaking down the blackberries with a spoon. Let the mixture simmer for about 15 minutes, then carefully strain through a sieve and discard the solids. Return the sauce to the pan. Whisk together the water and arrowroot powder and pour into the sauce.
Turn the heat to very low and stir constantly until the mixture begins to thicken. Do not let it boil.
Remove from the heat and use immediately, or let cool before storing in the refrigerator. The sauce will thicken as it cools. This will make more than you’ll need for the 2 servings of fries.