The first thing to say is that the coin is a beauty so doing nothing would be almost certainly the sensible option.
That having been said and as someone not all together sensible, I would probably have a gentle explore of the area
to see if the black could be removed/ reduced
. if you have a binocular microscope I would have a look at the black material under that and use a wooden cocktail stick to gently posh as one small part
of the black material if it is immobile... personally I would stop there, but if even a tiny improvement can be achieved with slow work
.. I would work
on it and try to clean it. My guess is that it is a silver compound formed by Bi/Tri metallic contact and it wont budge easily but it might just be "dirt"
FYI I have improved some of my Byzantine collection
by gentle cleaning I have made no difference to most and spoiled several when I have either rushed or used metal tools