blueberries

We're going picking this coming Saturday and hope to have a good deal of blueberries to play with. We have some great canning books, and I understand that a certain amount of sugar is necessary in order to preserve, but what I'm starting to wonder is, how little sugar could we get away with and still preserve? Is there any way to determine this? With Spirited Blueberries a little rum seems to be substituted for some of the sugar, and it really improves things, not only for the flavor of the rum but with the lower sweetness level. I'd love to basically do something similar only smush the berries and call it a spirited blueberry syrup or jam. (I guess if I didn't use pectin it would be syrup.) Is there any website about proportions appropriate for canning? How low can you go with the sugar?