Apparently your fitter was able to measure your club head speed with some form of tracking device. If 88 mph is the 7 iron speed and not driver, you have impressive speed. That's the only information us arm chair fitters have to to go on. It's off season to be arm chair quarterbacks. If the fitter took you through 3 different shafts or more, and the launch, height, and spin gave maximum distance and dispersion pattern, you need to trust your fitter. At that club head speed, the AMT Red would balloon too much for you, so Black doesn't sound like a mistake.

My swing speed is slightly lower than that, thus I use regular flex. I could probably manage using a stiff shaft, but my ball flight is kind of low to begin with, so the regular flex helps me get the ball up in the air more. For your case, I would test both and see which shaft produces the best numbers. It is hard to tell you exactly without seeing your swing.

I think that if your fitter recommended the AMT Black stiff shaft, then it's the shaft that gave you the best performance during your fitting (assuming you hit multiple shafts).

I have the AMT tour blacks in my 718 AP3's high ball flight but incredible shafts I have no idea of my iron swing speed I will hit either a 7 or a 8 if I am "amped up"

I used to play KBS these are really nice shafts

Based solely on speed a stiff flex would be the way to go. If you are 90mph+ with a 7 iron you open the door to extra stiff. Tempo matters, trajectory preference as well as ball striking consistency can play a role. Chances are your driver speed is in the low 100mph range. Definitely not a regular flex.

If you have faith in the fitter and you like the performance of the club then it's ok. My question is, did you hit the club with the suggested shaft? If not then you should try to find one to hit.

i think you also have to consider ball flight and trajectory when your talking shafts. As swing speed isnt the only factor. Also how your swing is at impact. Do you hit down? how much? are you flat? do you hit up? there are a lot of factors that go into shaft choice. But I know that the Titelist fitter take all that into consideration when they fit you. The simple question is, Do you get feedback from it? does it produce the shot you want? etc etc.

If you went to a fitter, why are you asking? Do you have faith in your fitter? Swing speed had very little to do with a true fitting. If he is a good fitter he can give you a lot more information than what your swing speed is, and you will have confidence in his recommendations.