The blackening of  toe nail can be fungal infection of the nail. You could have contracted it while playing  sports,  sweaty shoes,  using elses socks,  or even during a pedicure in salon. Avoid all the above situations. Keep feet clean and dry. Start applying Anti fungal nail laquer as Nail on or Iwin on the nail each day for 3 months as local antifungal and contact a dermatologist near you for oral medications  if more feet nails are getting infected. It takes 6 months atleast for the nail to recover and get a new nail.