Light-colored jeans, such as a stonewashed blue, gray or khaki, pair well with black shirts, helping offset the darker color. In general, pants should be a coordinating neutral color that focuses attention higher on the body and face because people tend to pay more attention to those areas.
Sometimes, wearing a black shirt with black jeans helps make a statement about a mood or helps wearers look a bit more sophisticated, especially when paired with a stylish jacket or shoes.
Some color combinations to avoid are black shirts with dark brown or dark blue jeans. There's not enough color contrast to make those combinations fashionable.