Well, Jim...
I blued an old Stevens .22 back in the early 70's using Birchwood Casey cold bluing. It still smells the same today even after wearing thin and going through dozens of oilings and cleanings. I blued several BP revolvers recently using Oxpho Blue. Same smell, different products (and different decades)
Can't speak for all cold bluing products, but that's two for sure. I believe it's one of the main chemicals, can't say which one.