There's a couple of different things that could be creating the black appearance of your tooth. It may be the tooth itself, or it may be the crown.
Crowns can be made from a choice of different materials. Sometimes a metal core is used with porcelain fired onto the outside to make the tooth look more lifelike. It is easier to get an accurate fit with metal than it is with porcelain, so in areas where it won't be seen the edges of the crown are sometimes done in silver-coloured metal. It can look very dark if it is not highly polished or once it tarnishes, so this may be the "black" you are seeing.
I think it's more likely that it's the tooth itself which is discoloured. A tooth which has had root canal therapy is no longer alive, and has usually had dressing pastes placed inside it before the final root filling was done. There are a couple of theories as to why root canal treated teeth go black. One is that components of the blood which used to flow through the pulp when it was alive can hang around and cause dark staining over time. There is also evidence to show that the antimicrobial and anti-inflammatory dressing paste used in root canal therapy can stain the tooth. Strangely enough, one study showed that in the presence of dressing paste, only teeth which were exposed to sunlight discoloured. Sealing your lips closed to keep your teeth in the dark is not really an option though!
Nobody really wants black teeth, so what can you do? If the discoloured area of the tooth doesn't show when you smile, it's probably best to leave it alone. The staining doesn't indicate a problem, it just doesn't look nice. If it shows in your smile and bothers you though, there are a couple of options. Unfortunately, it may involve compromising your crown. If the dark area is indeed a metal margin, you can talk to your dentist about the pros and cons of replacing the crown with one that has porcelain all the way down to the bottom. If it's the tooth itself that is stained, one option is to have a new crown made which extends a bit further down the tooth to cover up the dark area. Root canal treated teeth can also be whitened from the inside, which involves making a hole in the top of your existing crown to get inside the tooth and putting a whitening paste inside. Finally, depending on the clinical situation, it may be possible to simply cover up the darkened area with tooth-coloured filling material.
Unfortunately it's impossible for me to know which option is best for you without having seen your teeth or knowing the history. If it's something that's bugging you though, I'd recommend you go and talk to your dentist.
Thanks,