This is a placeholder because an independent scholar and art restorer named Chris Dobson has brought up the problem whether much European plate armour was blued or blackened before the sixteenth century (that is, it was heated evenly until it was coloured, and then either organic material was burned on to it, or it was rapidly cooled by quenching to lock coloured oxides to the surface). Almost all armour before that period has either been allowed to badly rust, or scrubbed back to bare steel, or both at some time after its working life. Therefore, its almost impossible to determine the original surface finish by examining surviving armour from the fifteenth century or earlier.
Shiny surfaces are extremely hard to photograph or paint, and before about 1430 artists didn't really try to depict the outward surface of things as the eye would see them. Many people say that by sometime in the fifteenth century, there are so many very detailed and lifelike pieces of art with dark armour that it was probably a common thing. Earlier its hard to decide how to interpret the art because "this is an expensive painting, use lots of gold, silver, and precious colours" was a thing that patrons asked. There is lots of medieval art where the artist clearly mixed up a small palette of colours and stuck with that so the work would go quicker. So when we see something like this:

its hard to make a convincing argument for why the first knight's surcote, the second knight's helm, the young man's tunic, and the old man's hair are all the same grey colour. You can be more confident with someone like Hieronymus Bosch in the sixteenth century who really seems to want the head of the crossbow bolt and the mittens of plate to look different.

Since the argument is in
an expensive book which arrives slowly by courier and has arguments which use a wide variety of evidence in ways that are sometimes convincing and sometimes not, the quality of the online discussion has not been the best (lots of comments by people who have not read the thing or have only read parts). For example, Chris Dobson is very optimistic about seeing things as authentic which most people agree were made in the 19th or 20th century (occasionally by cutting up medieval armour). I have to be careful how much time I put in so I don't get distracted from things that would pay in money or CV lines or get me out the door.
This thread is pretty good on polishing.