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.
Xanthan gum if you don't want to modify the flavour.
Or
Boil in heavy sugar syrup if you don't mind sweetening the sauce significantly.
Personally I'd do both, I imagine what you've created it BlackBerry juice without much flavour. I'd first start by boiling the berries in some sugar untill much of the water had evaporated and tasted exactly how I wanted (intense BlackBerry flavour) then if it required use a little xanthan gum to get the right consistency.
Best Answer
What you describe are definitely symptoms of coagulating protein. Not all proteins in quark are coagulated, else it wouldn't be creamy but rubbery like mozzarella (actually, more like cottage cheese, because it would still be grainy). My first guess is that the blueberry acid curdles the protein.
Blueberries are also known to act a bit strangely due to their high pectin content, I once left blueberry-banana puree in the fridge with the intention to freeze it to sorbet when I come back from work, only to find it gelled too hard to go through the machine. But I doubt that this was the culprit in your case, if anything, hydrating pectin would have bound the water and prevented the proteins from the curdling. Setting pectin would have created a mass which is firm, but not grainy, and without a separate watery phase.
As for the bitter taste, many fruits will create an unpleasant taste when exposed to dairy for a long time. Kiwis are especially bad offenders, but some melon cultivars are just as bad. I haven't noticed it in blueberries, but I don't know if I have ever mixed them. Also, there are different plants known as "blueberry", most of them just different cultivars, but the American and European variety are distinct species. It is possible that only some of them have this problem.
If you want to prevent the curdling problem, you can add stuff which will get in the way of the proteins looking for a buddy to curdle with. Fat and sugar are very good in this respect - this is why you can make lemon tarts pretty well, you don't curdle the custard despite the acid lemon juice just because the sugar content is so high. If you are trying to create a low-calorie snack, you can try gelling agents instead, which will not only reduce the curdling rate, but also sponge up the water when a bit of curdling occurs, leaving the final texture more pleasant. But it will also change the texture a lot, making it less creamier (the one creamy binding, starch, can't be used without cooking) and more like a jelly. Also you might get weird synergies between the natural pectin and the new binding agent, ending up with surprisingly firm food.
In short, the best option is to just eat it right away. Everything else has drawbacks, and is not a complete solution in the sense that it probably won't stop the curdling completely.