Black shade calibration ensures that your camera has clean and consistent pixel sensitivity across the image. It's a process that often gets overlooked, despite it being something that ENG camera ops of a certain vintage will have been used to doing on a regular basis. For instance when your camera moves from a warm interior environment into a cold exterior one.
For a newer generation of camera users used to DSLR style cameras, or who never enter the calibration menus, black shading will be an alien concept. In the video below, produced by RED, the company explains how to perform the black shade calibration process on its cameras. However the video is also of use to users of other makes of cameras, as the general considerations on how to peform the procedure, as well as the reasons why you would do it, apply across the board if your camera has the option available to it.