It usually means the nerve of the tooth has died. The roots may still be viable though so you may be able to have a root canal done and then the dentist can apply a bleaching agent to the inside of the tooth before sealing it to complete the rc procedure and this will make them appear whiter or closer to your natural tooth shade - or he may suggest they be crowned as root canaled teeth have a tendency to become brittle and break or fracture over time.
my dentist has already mentioned dentures for my upper teeth (they are getting very loose with the gums receding), and the one tooth that is turning black has a gum-line filling that you can see has receded lots as well. my guess is dentures will be in order shortly - don't even want to think about it!!!!!