The great thing is that blackberries have very little sugar, so one salad serving * is only around 6 grams of sugar total. (girlfriendisbetter.com)
I added about 7 tablespoons sugar total and may add more. (cookincanuck.com)
However, for my purposes when I calculated out the carb content of the whole bottle there was less than 3 TBS of sugar total so for me it was worth it to buy prepackaged rather than make my own in this case. (sweetpotatoesandsocialchange.com)