Messing with old steels is always going to be a bit hit or miss but fortunately most blades for leatherwork are on the small size so as Chuck said a MAP torch should usually be ok. I have a Bullfinch gas torch and it gets almost as hot as a MAP so is fine for small blades. A good starting point is a red-orange heat and then quench. Sometimes quenching in water, rather than oil, can give a harder steel but the danger is the shock from a water quench can warp or crack thin steel - happened to me on my last experiment.
And yes, the older saw blades are often better quality steel, although I bought a really cheap Chinese panel saw that couldn't cut a straight line but the steel appears to be plain carbon steel and it makes quite good little blades, so it turned out to be good for something after all.