Aaah, the black toenailThis develops from your toe hitting the front/top of your shoe when you run. I've heard of and have had moderate success with changing to a shoe with a larger toe box. Go to your local running store and let them know your problem. I had a pair of shoes that gave me black toenails during training and switched to a different one that never gave me a problem. So I think it is definitely a shoe-fit issue. My current model never gives me black toenails during training (even 20+ milers), but during the last two marathons I've gotten three (lost one completely, one is now hanging on halfway and the third is likely just going to have to grow out).
I haven't tried this, but maybe it will work for you:
http://link.brightcove.com/services/...ctid1325126871