I had exactly the same thing happen to only one of my big toenails, about 2 years ago. It began to go black, and it eventually became black all over. I saw my GP, and he said that I should send some clippings for analysis, which I did. Result: no infection in the nail. I had no other symptoms, and there was no apparent cause. I had no pain, no red skin, nothing else. I did nothing about it, didn't apply cream or anything. After almost exactly 12 months to the day, the blackness grew out. I have not had black toenail since.