I have a question about the recipe "Blueberry, Oatmeal and Flaxseed Muffins" from Merrill Stubbs. Would I have to tweak the recipe if I used frozen blueberries instead of fresh ones?
No, you should be fine -- I'd maybe use the blueberries semi-thawed so they don't streak the batter.
I am new to the website, however, I have made the muffins (1/2 mix) using a frozen berry mix. I simply placed the berries in the frig for a couple of hours to thaw, then tossed them in about 1 tablespoon spelt flour before I added the berries to the batter. The mixed berry mix adds a fabulous texture and taste to the muffins.
I always use frozen blueberries (or blackberries, or raspberries) as they will thaw wonderfully while baking. Putting them in frozen prevents "bleeding" and leaves them whole to enjoy with each bite. You decide - but try it and see/taste the difference.
Frozen berries work just fine; there is absolutely no need to thaw (or to toss in flour, if they've been properly quick frozen and are thus loose, not clump-y). I use frozen Maine blueberries all year, except when they are in season. of course. Same for raspberries.