In so many historical paintings in books that I have read through the artist usually colors unsheathed swords as darkish blue, gray or even black. This is not the color of bare steel.

Did they coat the steel in a protective layer? Some sort of paint maybe that resists rust and fouling on swords?

Interesting. Swords in reality are not blue. When i was a child i was drawing pictures of medeival battles and i remember i used tinits of blue to paint armour and swords. There is just no other way to paint the sword. And those days i hadn't seen any real manuscripts, blue swords were just natural in painting.

I have read that in some cases what you're seeing is tarnished silver.

The tarnished silver was on the page or art. They used the metal as part of the illustration i.e. gold or silver leaf. Thus over time the silver turns black.

Best
Craig

Some of the helmets were painted blue too, and mail had less colors. I assume it was an art style of the period.

Christopher, I have noticed the same thing, also with respect to armour. And I agree with you in that I think the references do make visual distinctions in some cases between a light grey (what I imagine to be bare steel) and a darker blue or black (which I happen to think was likely a finish as in blueing).
Old blacksmithing books from the last century almost always include a discussion or mention of finishes for iron and steel... There are as many "formulas" as there were blacksmiths, but the general theme which I take away from this is that in the past and potentially continuing all the way back to the medieval period, blacksmiths/armourers/swordsmiths would have considered a finish of some sort as just a natural part of what they did. Of course this is an assumption! But to me, given the obvious nature of the oxidation issues with iron, a finish for high value items would have been a pretty natural choice. I'm sure there would have been trends through different periods and in different regions, etc, but my guess is that there would always have been those who relied on oiling the metal and those who would swear by a finish (likely in conjunction with oiling, IMO).
I have a copy of the "Old Testament Miniatures" book, which is usually referred to as the Maciejowski bible and which has some fantastic images of late 13th century arms and armour (in the context of the Old Testament). Now I know that this is conjecture, but it ~seems~ to me that there are obvious distinctions being made between different hues of the metal. Of particular note is the constant distinction between blue and white. For example swords are almost always shown as having a blue center and white (or light blue) cutting edges. Now to some people this would simply be a way of indicating the shape of the blade, to me it could also be that the blade was originally blued and that the edges, being sharpened and used often, would very quickly become bare. There are also cases where helms are shown with certain parts (like bands on spangenhelms) white and certain others blue. This could be an indication that they blued certain parts for flair as well as for surface protection...
Other colours are also shown throughout, mostly with respect to helms; gold, yellow, green and red being prominent and I think the general consensus on this is that they were actually painted rather than being "finished". But there are also pages where some of the knights are shown wearing helms which are entirely white, while others beside them wear helms of a dark blue. This could either be white paint, or bare steel IMO.
I have also noticed paintings in other sources depicting the Hundred Years War (these I think, were mostly 15th century paintings) where the armies are shown densely packed in groups and almost all of the armour and some of the arms are shown absolutely definitively as black, however other items in said paintings are grey, grey-blue and light blue, etc... common colours associated with bare steel... yet the artist clearly chose to depict the armour and arms and black. I would have to find, scan and present references to support these interpretations, but in order to quickly respond to your comment, I simply wanted to say that I agree with you in that there certainly appears to be room for discussion on this issue...

I've often wondered if they might have used a process essentially similar to seasoning cast iron on Medieval swords. The color can vary according to what kind of fat or oil is used, how many layers are applied, temperature, etc and seasoning can be done at temperatures that wouldn't affect the heat treatment. It's a thin and slick coating that shouldn't have an impact on cutting ability but it would provide a worthwhile boost in corrosion resistance and it'd be easily maintainable by the end user.

Hmmmm.....
I guess medieval swords do not have a protective coating on the steel. Maybe that's the sky reflected on the blade that gives it the blue colour.

Pigments change colour over time. Just because it is blue today doesn't mean that it was blue when freshly painted.

It's important to note that as swords were prestige items for most of their history, as indeed many weapons were, they would have been maintained with care in the time that they were used. If they were stored away for years in an armoury, then they might tarnish and rust, but once taken out they would have been polished up for use again.

Those that could afford to purchase swords could afford to take care of the blades themselves or have someone do it for them; those who got swords in other ways would be unlikely to get another sword the same way again (picking it up on the battlefield) so they would take care of the sword they had. Coatings tend to be fairly thin and easily scratched or rubbed, unless they're an intensive modern process or something along the lines of the black finish that blacksmiths can render on iron.

So as a practical matter I very much doubt that there would have been any coating or finish placed upon the blade of most weapons other than oil or wax. There was no real reason to do so with the technology they had at the time.

Trying to use the colour in a medieval illustration to give us any details about the gear they carried is an exercise in futility. These things are not photos and can't be interpreted the same way.

The viking sword from Bath has what is believed to be an original deliberate chemical blackening of the blade.

[ Linked Image ]

I forget the details exactly, but the metallurgical report indicates the black finish is likely original and is not the product of environmental oxidation. The authors of the report speculate that it may have been created with tannic acid, or perhaps urine.

If I remember correctly, there is atleast one other "viking" period sword with a similar black finish, though it escapes me which example that is exactly at the moment.

The blackening of the steel is a natural oxidation process that occurs with any non-stainless blade (mean any ancient blade, by the way).
The color depends on the chemical allied elements that can be naturally presents in the steel because of the composition of the ore.
Also, the blacksmiths know how to play the heat treatment temperatures to obtain some colors of the steel. This can affect the physical characteristics of the blade in some cases, and make them unsuitable for use but remains fine for ceremonial use.

I would say that if there was surface treatments for iron and steel blades in use, we would have more sources than illuminations or painted sculpture to go by.

It would likely have been mentioned in inventories, sagas, in any military documents, laws concerning militia and their equipment (such as the gulating law), and maybe some mention in books like Teophilus: "De divers artibus" and as far as I can tell, there is nothing of the sort.

Also remember that paint and color in this time is decided by many factors, where realism is just one. You have to factor in:

Symbolism
Cultural conventions
Pigment availability
Pigment price
Pigment suitability for the medium it is applied on