If I'm specifically looking for sour blueberries, what can I do to best avoid the sweet ones?   If buying from the store, do I just look for berries that are more red?  Perhaps there is a particular time of year when they'll be more sour? 
You could try sugars with a lower sweetness value. I have used glucose, for example, to cut the sweetness of my granola recipe. It's about 75% as sweet as sucrose, while retaining most of the other chemical properties of sugar (so it still makes clusters).  The lowest I've seen is lactose, at 20%, though plenty of people are lactose intolerant so that might not be a great idea. Maltose and galactose are also pretty low.   This might help you get started: http://owlsoft.com/pdf_docs/WhitePaper/Rel_Sweet.pdf 