I got black toenail on my left big toe after just a couple times wearing a pair of running shoes that were too small in the toe box and the same size as I normally wear in street shoes. I learned a lot researching that -- one thing is that I need to buy running shoes at least 1/2 size larger than street shoes. It took nearly a year for the toenail to become "normal" again, since it was the big toe (it never fell completely off, thankfully), and I've never had a recurrence.
Google "black toenail" for much more information. New shoes are the first order of business, as others have said. Good luck!
Emily
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow