How to prevent pureed blueberries from gelling?
Blueberries, and especially underripe blueberries, have a lot of pectin. Blueberries have about .4g per 100g compared to apples which have .5g. As you suspected this is almost definitely causing the problem. Many blueberry jam recipes consist of just heating pureed blueberries with sugar and acid- no added pectin needed.

When you heated your pureed blueberries the pectins dissolved out of the cell walls and developed a negative charge that would keep them from gelling. The acid in your recipe, as well as the sugar, created perfect conditions for those pectins to re-tangle and gel.

Solutions to this problem would be:


Use riper blueberries- ripe blueberries are lower in pectin.  
Use less sugar
Add less acid


Obviously the last two suggestions are easier but will change the flavor of the recipe somewhat.

You could also add less fruit but, as you noticed, a little pectin goes a long way. I fear you would have to drastically reduce the amount of fruit which would be sad.

An interesting paper about pectin says in reference to LM pectins: "The presence of acetyl groups prevents gel formation with calcium ions but gives the pectin emulsion stabilising properties."  

Of HM pectins it says: "Acetyl groups prevent gelation and the DM within the group of high methoxyl pectins determines the setting temperature of a gel."

Other papers point out that the more acetic acid that can be extracted from a fruit the poorer the gelling properties of its pectin.

Wikipedia concurs that acetylation prevents gelling of pectin. Acetylation of salicylic acid to make aspirin is done with Acetic anhydride. If you could get your hands on this compound it seems likely it would drastically decrease the gelling ability of your pectins.

I included this last suggestion for the sake of the anonymous hordes on the internets because, as Acetic anhydride will react with water in the air to form acetic acid, it reeks of vinegar. And everyone knows that you hate that smell.