Older keris very often have uneven edges, if it troubles you you can even up the edges by filing. Personally, I would not bother dressing the edges of an old blade unless the edges were very severely eroded and/or the blade was of very high quality. The little bit of erosion that you have drawn our attention to is nothing. It is normal, just accept it.
Apolaki, this is a keris. Everything I can see about this keris is totally normal for a keris of this age and quality. Yes, the edges are uneven, yes, the colour is uneven. This is exactly what we expect with a keris like this. There is nothing wrong with it, it is normal.
I cannot refer you to any published works that can explain why heat-treated steel goes black. I guess it is simply because the steel contains carbon, the iron does not, when the steel is heated and then suddenly cooled the carbon at the surface goes through some sort of change. An engineer can probably explain the reasons, I cannot.
As to why you did not see this change in colour in other keris you have cleaned, there could be a number of reasons. Maybe they had not been heat treated, or if they were old blades, maybe the blade had been annealed and the steel had become soft, maybe the blade did not have a steel edge or core, maybe it did have a steel core, but the core projected around the edge by only a small amount and any colour change would be difficult to see, maybe only the tip of the blade was heat treated. Lots of "maybes" and a different maybe can apply to different kerises.
If you do not like the look of the blade as it is, you could always repolish it and then use cold blue that is used for firearms repair to give it a deep blue colour.
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