However the book states that an entirely new trapping can be added (YS 147), and I am wondering that if a new trapping is added how much can it impede on existing trappings? The example from the book suggests adding an Herbalist trapping to Survival, allowing the character to make declarations concerning plants and use them for minor first aid. The latter part slightly encroaches on Scholarship's Medical Attention trapping.
The problem with a "catch all" trapping is, that you can concentrate completely on boosting one while ignoring pretty much every other skill. My interpretation on the example in the book is this: If you want to know if a character wants to know something, in this case plants, you would roll scholarship (more precisely, the Answers trapping). A clever player could say "Hey, there's this herb, that can help with wound healing, we are in the middle of the woods, sure I can find some of it around here." and then roll scholarship for a declaration. Now the stunt lets you use your survival skill as a knowledge skill for plants (that is the Answer trapping moved for a limited set of circumstances), so the same reasoning still applies. Still the stunt itself does not come with a built in first aid. It might be used for it, but it is a GM ruling.
I admit the wording on that example might be a bit off.
Now how does that help you with your ninja problem? Bottom line is, trappings are still limited at what they can do. Bundling up 5 or 6 skills in the trapping of 1 skill should not be an option.
My apologies for giving a bad example as your solution is an easy fix but does not answer my question. If you do add a penalty to a stunt, can it give more than two shifts of effect? I know that the more specific the application the more shifts a stunt grants, but I'm talking about fairly broad circumstances where a stunt would normally give a +2. Again, I'm looking at the Listening stunt (YS 153) as reference.
I think the listening stunt works like this:
First of, it is a straight out +2 to listening rolls. BUT at the same time it places the aspect "listening intently", which is compelled to drop your alertness to terrible until you stop. The resulting fate point is spent on the same aspect, giving you another +2 on your listening skill.
Fix (OW146) has a similar stunt: "swing to the fences". An attack stunt can usually only add +1 to the attack, this stunt grants an additional +1. For that, the attacker gets compelled to take -2 on every dodge until his next turn.
Analysing the stunts like this might seem a bit convoluted, but I hope you get what I am trying to say. A bonus for a compel, the standard fate currency flow. The bonus should double the usual bonus of the stunt, but never grant more than +2. And the compel should be an actual disadvantage, of course.
I would still not create a blind fighting stunt based on this. To be honest, I don't really like the stunts above either, I would solve this directly by using aspects, declarations and maneuvers.