A.
Root canal does not guarantee you are not going to have a problem . It is a treatment of last resort to prevent the tooth from being taken out once the tooth is damaged. The procedure involves removal of the nerve tissue and bacteria from the tooth canal and filling the space.as the mouth is not sterile there will be some bacteria trapped in the tooth. Sometimes the dentist cannot fill the tooth completely due to obstruction. Hence you can still have problems, especially when your immune system is run down.
The darkness is due to lack of blood and nerve supply which was removed during the root canal.
My advice get an opinion on if the root canal can be redone better to get rid of the pain. If it can then once done think about bleaching the tooth or a crown