without knowing ahead of time what was used.

While I haven't done formal double blind studies I can relate this story:

Some years ago, for our mother's 80th birthday party, my sister asked me to make some cakes. I did. Had you been at the party that day, when dessert came around, you had a choice of chocolate cake with white frosting or chocolate cake with chocolate frosting, or another dessert she fixed.

Now, this sister had three young children and did the usual birthday parties and other holidays where she baked cakes. From a mix. Sister was used to people, even kids, taking a few bites of the cake and not finishing.

To her astonishment every single person that took a slice of cake ate the entire piece. And commented on how very good it was. I wasn't as surprised, because I was used to that response.

Again, at the risk of repeating myself, every time I bake stuff I am amazed and a little embarrassed at how people rave as to how good my stuff is. Heck, I even had one co-worker who said she didn't like chocolate, try whatever it was I'd made that time (probably my brownies) and just loved them. I'm guessing she'd never had anything not from a mix before.

I'm not claiming deleterious effects, because (even if I suspect they exist) I don't know what if any research has been done. All I can say is that (anecdotally which is such a dirty word) I've seen over and over again how people respond to the made-from-scratch items.