Although we don't use cups here the proportions for a traditional jam are equal weights of sugar and fruit. 1 cup sugar is about 200 grams, depending on the type of sugar. 1 cup blueberries is about 140g. And you were using 2 cups of sugar per cup of fruit! That is more than twice as sweet as a classic jam, let alone a low sugar one.

You could try this question on the Harvest Forum which is specifically for canning info. Run your recipe by the Harvest people and see what they suggest.

Here is a link that might be useful: Harvest Forum

Hi Flora - no wonder it was so sweet! Thanks for the link, I'll check over there - Eric