There are some permatex remover chemicals out there, don't know what effect it would have on the bluing. You could pick up some B/C blue and test it on a piece of scrap steel by bluing the steel then applying the chemical.
Somehow I doubt heat would work so it seems you're left with a chemical remover or a flat edge to pry/scrape.
The good news is it's not that hard to re-blue steel.
use a quality gasket remover, it should not harm the bluing unless its left on there for extended periods (2+hours). You may still need to use a box cutter type blade to scrape the remaining film off of the slide.