Military issue Colt M1911 types were heat treated at the front only in order to strengthen the impact area of the tubular area surrounding the recoil spring where the slide stops against the frame upon recoil. It would prevent the slide from peening or distorting from frame impact in that area. They did not heat treat/harden the slide behind that point perhaps for production speed or cost, but it does happen to keep the rails that ride the frame from cracking if someone squeezed the slide to tighten it up. Keeping the rearward portion softer may not have been deliberate, but hardening the front was. In any case, those slides are easier to squeeze to tighten the slide to frame fit than slides that have been completely hardened.
Civilian slides were heat treated full length to make the bluing an even color throughout for cosmetic reasons.
It does, however, make a two-tone blue or Parkerize on the Military slide. Unless you were to re-heat treat the entire slide to even out the hardness, simply reblueing the slide will not change it, it will always blue differently in the front.