That black stuff in the lettering may be silver sulphide. It is very difficult to remove without detriment to the coin as a whole. Best left alone.
It MAY be lessened by gently scraping with a sharpened match point, but I seriously doubt it. I have tried and failed with an Indian .925 silver rupee of 1909 in similar circumstances.
My coin is close to mint state, but the black blotching is so bad, that I just threw it in with my junk silver. For me, it was certainly worth the risk, but not so in your case.