It could simply be that black paint is cheaper... But I believe that the external coils on the back of refrigerators are painted black for the following reason: There is a rule in nature that says "a color that is better at absorbing heat is also better at radiating heat." Here is how Encyclopedia Britannica puts it:

...The rate at which a body radiates (or absorbs) thermal radiation depends upon the nature of the surface as well. Objects that are good emitters are also good absorbers (Kirchhoff's radiation law). A blackened surface is an excellent emitter as well as an excellent absorber. If the same surface is silvered, it becomes a poor emitter and a poor absorber.

Since black is a good heat-absorbing color, it is also a good heat-radiating color. One of the things the external coils do is radiate heat, so they are painted black to help that process.