What's the difference between fresh and frozen blueberries for baking? A recipe I have has a review that states that frozen blueberries in a blueberry muffin are absolute garbage and only fresh ones will give you that desired taste. I have also seen this answer that notes that frozen blueberries that are thawed will give a better taste and composition in a blueberry muffin.

My wife insists that fresh blueberries are the only real way to make blueberry muffins, and that frozen blueberries are usually rotten, and aren't as real (she cites the fact that when baked, frozen thawed blueberries come out blue, but fresh ones come out purple.

Is there any real life merit to her claims? From a baking standpoint, what's the difference between frozen and non-frozen?
When you are making blueberry muffins, frozen and fresh blueberries will give different end results. Frozen blueberries will almost "melt" when you bake the muffins, because the skin becomes more fragile after freezing. If you use fresh blueberries, they will be more intact after baking and will sometimes still "burst" when you bite into them. Which of these outcomes you prefer is entirely subjective (though obviously frozen are better).

As for the taste, some blueberries will have more taste than others. If they are grown out of season, they will most likely be flavourless. This goes for both fresh and frozen blueberries. I've found a brand of frozen blueberries that I use for muffins that give a have a nice taste year around: others may be terrible.

For the remark your wife made about frozen blueberries being rotten: I don't know where you live, but that is most certainly forbidden in the US and Europe, and probably most places where frozen blueberries are available. That is the only part of her argument that was most definitely wrong. Everything else is subjective or depends on the specific brand of blueberries or even the time of year.