So, I've seen many pixel arts in my day, and one things that is fairly inconsistent is the block border around the sprites.  Does it all boil down to artists choice?  Or is a black border a clear sign of a beginning pixel artist?

It might even be totally dependent on what look you're going for.  If you're doing sprite work for games, you might not want your character sprites to blend in with the background, so you add that black border to make sure you can always clearly see the characters.  And in a piece of pixel art, I'd assume that a black border wouldn't really be necessary.

Can anyone fill me in on the rule of the black border?