Are the leaves and stems themselves black, or is the black covering the surface of the leaves and stems, but they are green underneath? When did the black show up? Most boxwood diseases turn the stems and leaves brown rather than black, but they could eventually turn black as rot sets in. In the other case, a surface coating of black could be sooty mold, a fungus that grows in secretions from insects such as aphids and scales. Check the plants closely for these insects and control the insects to get rid of the honeydew in which the sooty mold grows.