My neighbor brought over a fresh batch of blueberries.  Some are sweet and some are sour.  He told us to come over and pick some whenever we wanted, but I don't know the right time to pick them so that they taste the best. Any advice? 
In the case of most bush berries (blueberries, raspberries, blackberries) depth of color and ease of picking are the most indicative signs of ripeness.  The berries that are on the sunniest side of the bushes will ripen first.  The greater more sun exposure they receive the quicker they ripen, thus the reason that a grouping of berries even in one area will ripen at different rates.  Many people say that the larger berries are sweeter but my experience is that is more anecdotal than preciese.  Most fruits will be more plump with access to more water during the growing season but that doesn't necessarily make them sweeter.  Grapes that have to struggle generally tend to produce better wines with more concentrated flavor because the natural sugars aren't watered down by high quantities of water.    To reiterate, look for deep dark color and ease of picking. 