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?
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I too love sour blueberries. I usually shake the punnet gently in the shop, and if they 'rattle' like they're hard, they will often be fairly sour. If they just thud around quietly, they are probably ripe and sweet.
The same theory can be applied to picking blueberries - go for the firmer ones.
In the UK, the time of year that is best for sourness is around May/Early June for British-grown blueberries. Slightly earlier for Polish or Spanish berries (I find Polish are often the best - tart but full of flavour)
Blueberries don't continue to ripen after picking so finding those that are blue and tart can be a challenge. If you see berries that are slightly purple or red, they may have been picked early and be a bit tart.
If you're in an area where blueberries are raised, you might visit early in the season, which is late July to mid August. Picking your own might do it, or finding a wild blueberry patch. And if you're really into them and have the ability, consider planting your own bush; early Spring is the time to do it.