Birchwood Casey Blue & Rust Remover. I believe it's a phosphoric acid based product.
I have no idea what it might do to any case color under the blue, and for that matter, I have some doubts any case color remains, because bluing is not an applied coating like paint, but rather is an oxidation process on the surface of the steel, just as is the case coloring.
Thinking out loud, bluing is very, very thin, whereas case hardening can run .004"-.007" deep into the steel. Maybe you will luck out and reveal it stripping the blue off.
If you don't like what you have, then you don't have much to lose spending about $7 for the Birchwood product and seeing what happens.