If you are looking at a visible image, chances are it is at night or during the twilight hours. Because visible imagery works by measuring the amount of visible light reflected back to the satellite sensors, overnight when there is no visible light, the images appear black. Around sunrise and sunset, when the light is very dim, the satellite imagery will appear very faint and will be hard to resolve detail on.
If the image that you are looking at is infrared, there is probably a problem with the satellite. Due to technical malfunctions or routine maintenance, sometimes infrared images are sent from the satellite that have large areas of no data on them. These will show up as partly black images.
18:24 EST Parts of Sydney and the Blue Mountains have been blanketed by large hailstones as a severe thunderstorm moves across the area.