Blueberries are acidic and can very well, most recipes, even a cold pack, requiring only a water bath. Corn starch, if it's in your recipe, might be a problem, more from a quality than a safety perspective. It's best, of course, to follow a recipe from a reliable source.
If this is a dessert sauce, there probably isn't enough acid. I would just freeze it flat in freezer bags which would also preserve the fresh flavor. I am freezing everything these days. :)
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