There are methods used to protect metals which also reduce their reflectiveness. Usually with either a matte grey or black. In a painting sense, you use Non-Metallic Metal first, then True-Metallic Metal blade edges and scratches.
Heat Treated Metals
Adventurer comes from a family of dragonslayers and his family sword could have seen this kind of discolouration over the years.
As I mentioned above, this technique works on most base coats. In this case, I've painted a NMM base for the sword with shadows and highlights.
To get the best effect, apply them all wet so they mix together, so gather the washes ahead of time: Red, blue and brown.
In each step, I take get a bunch of the wash on my brush and dab it on the metal. Blue in the first step and purple in the second. Notice though how in the second step the blue and purple mix together where the blobs meet. To finish it off, I use a little brown to add some wear. In the final photo with the Adventurer, you can see how it looks once it's all dried.
Parkerizing / Phosphating
This is the process of treating a real metal so it actually has a NMM finish. Other than for stylistic reasons, this might be done to weapons so they don't reflect light, something that would be a benefit for... discreet users of weapons like this below imaged Half-Ogre Assassin/Chef.
Painting this doesn't actually take much more beyond a NMM paintjob, which in this case was a cool-grey approach on his daggers. But because this is a surface treatment, the sharpened blade edge and scratches should appear shiny. Paint TMM along the sharp part of the blade and some lines or dry-brushing from that edge to create scratching.