I just did a rust blue job on an old FN 1905 (manufactured ~1913). Probably going to do another pass or two to even the color out on the flats. Oddly, curved surfaces don't need another pass.
The first pic is after boiling on the second pass, the next is after carding, then reassmbled.
The last pic is a Japanese Type 99. The bolt looked pretty much like the action before boiling for 45 minutes and carding. It's later-war (1/44) so not last-ditch but the manufacturing details were getting a bit scruffy. But the original finish was still hiding under all the rust! Barreled action will get cooked after I dig out the big tank and burner...