Gunnut is right on. I've been using Brownell's bluing salts for years. When I do a cast frame or receiver, I just add Brownell's Oxinate "S". The cast will take a nice blue, otherwise you get a plum color. "S" is a real pain to use and it's expensive. You have to bring the salts up to boiling, add the "S" then cool down and reboil a few times. Most gunsmiths don't want the hassle so they turn down cast jobs.
Most liquid cold bluing will work on the '94 receivers. Yes, it's a bandaid and won't last like hot blue, but it does make them look nice and will blend in with the original finish.