When discussing the color of Tahitian pearls (or pearls in general) the term "black" is used to describe pearls with dark body colors. It could be blue, purple, green, brown, silver -- but never a true black.
In addition to the body color, Tahitian pearls can display an amazing range of overtones (especially the circle and baroque shapes). Traditionally the most sought after (expensive) Tahitian pearls will have a dark green body color and peacock (think oil slick) overtones. Pearls that look something like this: