As for blueberries, I think the peak season for them now has past. I like the local ones from Parlee Farms in Tyngsboro, which area bigger, firmer, and still juicier than anything available at any supermarket (as I buy some from various supermarkets too).
In the Lowell/Southern NH area, I find MB corn to be dried out a lot. I highly recommend Peter's Farm in Salem NH, which is 20% cheaper than standard MB and Hannaford pricing. They are a little high on corn at $5/dozen, but bargains include romaine lettuce for $.90 a head, large bunches of beets for $1.50, huge cukes at $.40 each or picklers for $.85/lb, and there's much more.
As far as fish goes, I have been going all around. The fishy smell at MB turns me off and I find the quality at the ones I've been to in the Greater Lowell area is questionable (especially the Fletcher St location. Hannaford pretty much is very competitive in terms of price, and does match many of MB sale prices. I do recognize regular pricing at Hannaford can be about $1/lb higher than MB; but I really prefer to buy sale fish anyway as that does move more. Stop and Shop and Chelmsford has been keeping a nice fish counter together and I have been satisfied with everything I got from there, including they have had sea scallops on sale for $10/lb a week after a get a $10 coupon for your next order when you buy $50 in gift certificates of your choice. Bought $100 in GCs to Amazon (which was used fast) to get 2 lbs for free. Some of the stuff is extremely competitive if you come through with a decently sized basket, and most notably, they have boneless skinless all natural chicken this week at $1.99/lb, which otherwise costs $4-7/lb (what is commonly on sale at any store at $1.99/lb is not all natural). I do note that they have some weeks in which they do not have anything reasonably priced in terms of fish.