Why are my son's teeth black in colour?

My son is 3 years and 7 months old. Four of his teeth in the upper front section are in a very bad condition. They look as if they are rusted and appear black in colour. They also bleed at times. At times, during night he uses his feeding bottle. He used to take a lot of juices at the age of 2 but now he doesn't take any. Please tell me when these milk teeth will go, will the new ones also be the same? Please advise.