How does one GIVE ANY belt above white? If you were an instructor, you'd only give it to someone that you KNEW was consistently capable of performing at that belt level, when rolling/competing against others with that same colored belt. Giving someone a belt means that you vouch for their skills, now and forever, and that you're comfortable with them representing you and your particular type of instruction.
That's what makes BJJ so much more legit than other martial arts that use belts. There IS no "test." You just gotta prove to an instructor that you've got what it takes. And that can't be proved to an instructor in just one day, or from a visit here and there. Don't get me wrong...you may very well be capable of performing at blue level at any and every competition/class/open mat session. But that doesn't entitle you to a belt. Of course, you can go out and buy your own blue belt. But only when a qualified instructor GIVES it to you does it mean that it is LEGIT in the eyes of other BJJ practitioners.