Noticed that this post received a lot of "stupid" tags and I think it's worthwhile to mention that this question wasn't any more stupid than most that have been appearing on this site recently, in my opinion.
Unless you're in a pitch black setting under a moonless sky, black cloth will stand out, being considerably darker than it's surroundings. Navy blue blends in much better outdoors at night and in more dimly lit settings. Darker earth tones are helpful when attempting to not look filthy, as they don't show dirt as much as white or beige cloth would. Though white and beige absorb far less sunlight, which could be advantageous in some situations.