blueberries have a fair amount of natural pectin so if you do open, recook and reprocess, you might bring the jam to a boil again, make sure it's up to roughly 220 degrees and run a spoon or plate test. i rarely use pectin as i feel it leaves a slight aftertaste and i prefer jam w/ no wobble at all, so my bias would be to recook and test before adding in pectin. AJ is right that jams can take a while to settle in, but you should have a clear sense by now if your jam will reach your desired firmness.
another idea would be to add in some natural pectin- a stalk of rhubarb, an apple... just some thoughts.
I might wait a day or two though before doing it, because many jams get firmer over the course of a few days or a week. Put it in the fridge until re-processing. Also, I sometimes have the opposite problem - jams made with plums in particular can end up too firm, where the fruit's natural pectin is particularly high. I've been known to hold my processed very loose jams to see if I have any extra firm jams, and then mix the two, heat through to boiling for a minute, and then process. I've had some great after-the-fact combination jams, with a perfect consistency, made this way! ;o)
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another idea would be to add in some natural pectin- a stalk of rhubarb, an apple... just some thoughts.
http://www.pickyourown.org/how_to_fix_runny_jam.htm