How to pick out blueberries?

So here's my situation, I don't really care that much for the sweet blueberries, but I really like the tart ones. Whenever we buy them, they come in the little plastic carton so there really isn't any way to pick and choose which ones you get. But once you get home and open it, is there any way to know which blueberries are sweet and which are tart?

My first thought was that the riper berries would be sweeter. This is the case for pretty much all the fruits I know about, so I guessed it held true for blueberries as well. So...first thought, I figured that the larger ones would be the riper ones. I tried a few of the large and a few of the really small, but no success. They were all mixed pretty evenly. My next thought was that the bolder colored ones would be the sweeter, riper berries. It works for apples, oranges, and bananas so why not.... After about a dozen of each (bold blue and that sort of grayish blue), that too didn't not work. My final thought was that it might have something to do with surface tension, like an apple, cantaloupe, or banana. The softer the surface, the more over-ripe the fruit. So I looked for and tried a mix of soft and taught (for lack of a better word) berries. I had hoped this worked, but no success.

So, has anyone figured out, or knows how to find the tart blueberries? Thanks in advance.