Like all sage varieties, Black Sage or Mugwort is a form of artemisia, whose name comes from the Greek goddess Artemis (Diana). These plants are traditionally used for cleansing ourselves and our spaces (home, work) of astral parasites and negative energies.
We prefer mugwort over white sage because mugwort grows easily abundantly; white sage is being over-harvested by unethical suppliers (they often steal it from restricted sacred lands).
The most common way to use mugwort is in this form: dried and bundled sage is ignited on the end, then the flames are blown out, allowing the tips to smolder and release plumes of smoke. The smoke is then distributed over the area to be cleaned, in accompaniment with prayer. You can then dip the hot end in water, saving it for later use. In this way, a bundle will last a long time.
"In their operations of elemental magic, ancient magicians used smudging with laurel branches, mugwort (artemisia), rue, sage, pine, rosemary, etc. Such plants were burned on burning coals. This observance is magnificent. The air is filled with the smoke of plants, the exorcised fire will reflect the will of the operator, and the subtle forces of nature will listen and respond to him." - Samael Aun Weor, Secret Doctrine of Anahuac