Shadows are Silvers who can bend light. They can become invisible by manipulating the light around themselves. Their ability can also be extended to make others invisible. They can also blind someone by creating a light explosion or by keeping light from reaching people's eyes. The more powerful ones can even concentrate light and make laser beans. They are usually used as spies.
"Shadows are elementals, manipulators of light. Their abilities can be used to darken or brighten a room, for example, or even render themselves invisible to the human eye. Like silks, shadows are valued spies in most Silver armies and courts. Noble shadow families often elevate themselves through political scheming rather than wealth or marriage." - On Silverblooded Abilities by Victoria Aveyard