Why is the serial number blacked out on this device?

Because the serial number is unique to every unit manufactured, and any exposed serial number could be used to uniquely identify a person or exploit his resources, such as a fraudulent warranty claim. It's the same reason an image of a sample credit card will have a blacked-out or invalidated number on it. This does not mean that the device you are going to get will have a blacked-out serial nimber.