Many people have an image that Root Canal Therapy leads to black teeth because so many people notice when an endodontically treated tooth gets dark. After all, you don't see all those which stay the normal color. Occasionally a tooth will darken after endodontic therapy. It is especially noticeable when the enamel layer is very clear since the discoloration is in the dentin layer of the tooth. See anatomy of a tooth.
When the internal layers of the tooth are stained there are a few approached for treatment. Traditionally internal or "walking" bleach was the treatment of choice. A cotton pellet of a bleaching solution was placed into the nerve chamber of the tooth and sealed with a temporary filling. The idea was this would bleach the dentin layer better, since the solution is in direct contact with the dentin. The cotton pellet was changed until the tooth was an acceptable color.
Some dentists have tried opening up a hole on the back side of the tooth and having the patient use a home bleaching tray and syringe the bleaching solution into the hole. Once the tooth is an appropriate shade, the patient returns to the dentist to fill the hole.
In my experience, if the tooth is only slightly dark, bleaching alone can work well, but in many cases the bleach just doesn't get the tooth back to the correct color to match the adjacent teeth. Bleached teeth also have a tendency to relapse in short order.
Because of the inadequacy of the bleaching techniques, many doctors have placed crowns to restore the correct color. While this is effective, in my opinion, it may also be overly aggressive as far as tooth removal. Sometimes there is no alternative to crowns if too much of the tooth is destroyed already, but more often than not there is an alternative. Six years ago I developed a technique I call "Internal Bonding" in order to lighten root canal treated teeth. In this technique, the tooth is hollowed out from the back side until all of the dark dentin is removed and only the enamel layer is intact. Then composite resin bonding of the appropriate dentin shade is bonded inside the tooth.