> The really good thing is the idea of a blacklist though
It's not though, because untagged items are assumed to be suitable. This becomes an issue for people with health-related diets e.g. if you have serious food allergies you want to be very very sure your foodstuff does not contain the allergen. So a tag/categories system should have "suitability tags" (whitelist) in much the same way restaurants will have marks for vegan or jain/ahimsa or halal items, not for the opposite.
I'm replying to a comment which specifically mentions gluten. Gluten is an allergen, and hype aside there are people for whom gluten is an actual health risk.
It's not though, because untagged items are assumed to be suitable. This becomes an issue for people with health-related diets e.g. if you have serious food allergies you want to be very very sure your foodstuff does not contain the allergen. So a tag/categories system should have "suitability tags" (whitelist) in much the same way restaurants will have marks for vegan or jain/ahimsa or halal items, not for the opposite.