Agree with your point but the way you got there doesn't add up.
If the experimenter knows which are which and the subject doesn't, it's only single-blind. The disadvantage of this is that the experimenter might inadvertently reveal some clue to the subject as to which is which. That would make the experiment's conclusions more suspect than double-blind.
I do check the ingredients first now, but previously it was all reverse engineering from getting a headache to going back and seeing what I ate. I'd happily do a study in a lab, if it meant published research was going to be put out. The sinus headaches are miserable enough that I'll never be doing it just for interest.
Chocolate is a no go because I've cut out sugar and caffeine as well. I mostly eat meat, non-gluten carbs, and broccoli.
For MSG, I don't know about it ahead of time. If I do, I don't eat the food. For example, went to kbbq, ate some plain meat, rice, and egg, got a headache after leaving. Next time I was there I asked about MSG because it was the most likely trigger to be used. Turns out it was in the steamed egg and the salt and pepper mix. Avoided those and felt fine for multiple visits after.
I think this is a blind way of discovery because I didn't have assumptions about the triggers when I started trying to figure what they were.