I pre-treat with my laundry detergent. Scrub by hand with that, then spray Shout Advanced on and scrub again. Then I spray some more Shout on and wash with the rest of her laundry. I just got beet out doing that so maybe it would work on blueberry? Just don't dry them if it's still stained after the first wash.
Cold water, scrub it with laundry detergent, let it soak for a while (maybe an hour?), rinse. If it's still there, soak the stain with hydrogen peroxide. Repeat the above steps. There is a SLIM chance that the peroxide would damage the fabric, but it should be fine on cotton. Keep repeating the steps. And most of all, DON'T DRY IT IF THERE IS ANY STAIN LEFT! Good luck!
I have used oxyclean on blueberry stains and it's worked! I fill the washer and put in the detergent and oxyclean with the load of clothes and let it soak for about a half hour before starting it back up again. I am an oxyclean believer!