I think that Japanese sword people call this black coloured corrosion something like "inert rust", or "inactive rust". They do not seem to want to remove it on sword tangs. I truly do not know if it can cause further damage or not, because I have always taken everything back to clean metal, no matter what colour the corrosion is.
I normally use magnification and bright light when I'm cleaning away rust. Direct sunlight is good, but I also use very bright artificial light.
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