I have been making jello from various fresh and canned fruits, to use up a couple of boxes of plain gelatin that are in my cupboard. One packet thickens 2 cups of liquid and I have not had problems until now, whether I added boiling liquid to the powder or mixed it with cool liquid and brought it to a boil in the microwave. I had fresh blueberries, Trader Joe's organic blueberry preserves, Splenda, and Canada Dry diet ginger ale. I bloomed the gelatin in cool water, then nuked it - didn't watch closely, so it boiled over but just a little. I didn't lose more than a half tsp. I stirred it, then stirred in some preserves and Splenda, and after a few minutes, ginger ale to reach the 2 cup mark on the cup. When it cooled to room temp I poured it over berries in individual dishes. I had drizzled a tiny bit of agave nectar over the berries first. The jello didn't set as firmly as I'd like, or as it has in my previous efforts (raspberry, peach, cherry).
Are blueberries one of the fruits, like melon and fresh pineapple, that inactivate gelatin? Or was it the ginger ale?