I think there are two different ways to get a blue belt in Brazilian jiu jitsu.
The first is by being too badass to remain a white belt. Tommy is a good example of that for me; somebody whose skills as a white belt simply became blue belt level and almost forced the promotion. In that case, the promotion was in recognition of something that had already happened. As the kids say, "congrats, long overdue."
But the other way of getting a blue belt, it seems to me is the opposite. In the second case, the skills aren't there in some key way, mostly a lack of consistency, but there is enough of a lack of consistency to make instructors and higher belts wonder from time to time if so-and-so really knows what he or she is doing, after all. At the same time, the white belt has learned a great deal of jiu jitsu, enough to frustrate or even overwhelm other white belts of similar or less experience. Every now and then this white belt might catch a higher belt with a submission or sweep--but it usually the white belt's "best" move every time it happens.
Awarding a blue belt to this kind of white belt seems to serve a different purpose. Here, it is almost as if the blue belt is a sort of challenge, a shove off-balance, a new standard to rise to, a set of expectations wrapped around your waist.
In reality, both are probably at work to some degree. And invariably the majority of people who get their blue belts think that they got it too soon. But my suspicion is that not too many of the latter category--the "social promotion" blue belts--end up making it to purple belt. In that sense, blue belt serves as a sort of refining process, where those of the first type finish developing a basic jiu jitsu "game", while those of the second type find one--or stay blue.
