Some very good points, here, JC!
I agree that the flight suit has faded to a more gray than blue color, naturally. However, it's really not up for debate what color the flight suits were dyed, originally. They were all dyed with the same bath. Now, sure, it's possible that one particular suit was dyed a different color, but it's highly unlikely, especially since you've seen firsthand what can happen to dyed material over time.
It's also apparent that the dye used was blue-based, rather than black-based.
Three points:
1.) Which version of the suit has Hot Toys tried to recreate, here? The suit as it appeared on set in 1982 (or when it was created in 1978-79), or as it appears today? If we're going for on-set RotJ, bluish would be much more accurate. If we're recreating the suit 33 years later, gray would be more correct.
2.) I don't recall if you were part of the discussion a few pages back, but it's obvious that this figure is not intended to recreate ANY on-screen version, regardless of the fact that it is labeled RotJ. The helmet paintwork matches exactly ZERO of the screen-used helmets, and neither does the chest armor. So this is intended to be a more idealized version of Boba Fett. Hot Toys was not looking for screen accuracy, because if they were, they would have weathered the helmet and armor properly, and they wouldn't have included the ESB sidearm as a bonus piece (I maintain that an ESB-painted helmet would have been a MUCH better exclusive).
3.) Keep in mind the lighting that the figure has been photographed under. Here's an in-hand picture that looks much more gray than blue. So perhaps the suit is not as blue as it appears in some photos.