The sodium hydrosulfite (aka sodium dithionite) is a reducing agent so perhaps the acid stains are a form of iron oxide rust from some iron ore particles in the concrete, possibly from the coloring of that stone (maybe from iron sulfate). Basically, it sounds very much like stain removal from plaster which is really concrete (i.e. it's not really Plaster of Paris which is calcium sulfate -- pool plaster is really concrete which is Portland Cement with aggregate that is finer than in normal concrete).

The sodium hydrosulfite is a particularly strong reducing agent in basic/alkaline conditions (i.e. at high pH), though the product itself is acidic (according to the MSDS).

I don't see a problem using it, but of course you can try some on a small area first just in case.