Black Toenail Runners and team players who sprint are prone to black toenail. The is really bleeding in the nail bed caused by pressure from your toes jamming against the inside of the shoe. In the long run, once the pressue is gone, it may be very black and funny looking, but it is not a big concern. If you are unable to live with or relieve the pressure yourself, you may wish to see a doctor who will provide that service for you. In the absence of pressure, and if no permanent damage was done to the nail matrix, the nail will eventually grow out without the black hue.
Thick nails are often the result of pressure to the nail matrix.
Just as skin calluses, the nail matrix produces a thicker ail in an attempt to relieve the pressure on a prominent area. The thickened nail plate may then pose extra pressure itself. Usually, however, a thick toenail is there to stay. If it is unmanageable, a pedicurist can be helpful. Similar fungi can invade the nail bed and matrix and cause dystrophic thick nails.