The very darkest part of a shadow is the umbra. It's where the light source is completely blocked by the object in front of it.
You'll most often encounter the word umbra in astronomy, along with penumbra and sometimes antumbra. During a lunar eclipse, the very darkest, inner part of the Earth's shadow is the umbra, while the lighter shadow at the edge is the penumbra. The antumbra is the bright ring you can sometimes see around the shadow. Umbra is a Latin word that means "shadow."