making sugar free blackberry jam

Near our workshop there are huge blackberry bushes, and yesterday I picked a pan full of ripe ones to try and make jam without sugar.
I am experimenting with stevia, which is a good replacement for sugar.
I cooked the blackberries with a small peeled apple and some lemon juice, and instead of a mountain of sugar I put a teaspoon of stevia in it and 4 grams of agar agar to thicken the jam, and cooked it down a while.

Taste is very good, now I am curious if it will preserve as well as with sugar...

Update: Stevia does not preserve like sugar does, I suggest putting stevia-based jam in the freezer in ice-cube trays and to only defrost the portions you need. The benefits of not eating lots of sugar are, in my opinion, still worth the effort!