What is seen as blue surface is nothing else than rust, i.e. iron oxide. Thus, it shows the same behaviour against chemical agents.
The blue appearance (instead of rusty brown) is due to the oxide layer being very uniform and thin (sub-µm). The color of such an oxide film depends on its thickness, which in turn is a function of the temperature used for treating and the duration (diffusion of oxygen from the surface into the iron matrix).
As Bernhard has said, the oxide film is very thin, so thin that it produces colours by an interference effect, like a soap bubble, a butterfly's wing or some beetles' carapaces.
Hi Bernhard and Graham. This will be one lesson well learned.
I'm highly impressed, never heard the hand bluing effect explained like this before. I have seen many forms and many colors of iron oxide even blue iron oxide but never in my wildest dreams would I think it could be the blue on watch hands. I have known for many years that you get a certain color of metal by heating it at certain temperatures, but I didn't know until now that it's all merely a thin layer of rust.
I was thinking the hands had blued all the way through and all I had to do was clean them and touch them up a bit even though I have seen several videos of heat bluing watch hands and saw them take off the blue for preparation, so why that didn't stick with me
The reason I used lemon juice, just a few days ago I found on this forum another guy used it to clean a rusty hairspring, even said it took off the blue. Again, it didn't dawn on me the spring and hands might have been blued the same way and the blue would come off
Anyway, for a long time I have wanted to give the bluing a try and now by way of another one of my happy accidents I have an opportunity. Since the brass shavings is one of the preferred methods, going to order me a bag and try that route and put my other idea aside for a while. And if they don't turn out good, I know how to get the blue off to try again.
So thanks guys for taking the time to guide me I really appreciate it. I feel like I'm in the middle of an advanced chemistry class but my level of understanding is stuck in grade school