In my opinion, simply it is black just due to the tradition, as everybody does the same, and everybody gets used to that color.
The same question you may ask why PC/monitor/keyboard/mouse/other PC-related stuff have the ivory color, while laptops are black or darkgrey.
Recently, manufacturers seem to change color/geometery of some devices for easy regconization: optocoupler are ivory, some Japanese manufacturers make their TO-220 packages dark-green, the same with some European manufacturers make their TO-220 chambered.
nguyennam