Getting a black belt in Brazilian Jiu Jitsu is really, really hard. It takes an average of about 10 years of dedicated training, but sometimes as many as 15. A black belt in BJJ represents serious dedication to the art and incredible technical proficiency. Even achieving the rank of blue belt (the first belt after white) usually takes several years. This separates the ranking system in BJJ from that of most other martial arts. If your friend tells you that his 60 lb niece got her black belt in grade 4, it wasn’t in Brazilian Jiu Jitsu.
In addition, Brazilian Jiu Jitsu is a fairly young martial art. Today, practitioners can almost always trace their lineage back to the originators of BJJ. For example, my BJJ lineage is:
Mitsuyo Maeda → Carlos Gracie → Helio Gracie → Pedro Sauer → Mark Johnson → Me
The difficulty of obtaining black belts and the short lineages associated with BJJ means that it is nearly impossible to fake your rank, especially at the black belt level. People have tried this before, and they are quickly (and often very publicly) outed. The difference in skill level between a new white belt and a black belt is immediately recognizable by somebody even slightly familiar with BJJ, and moreover, it is exceedingly easy to verify somebody’s lineage. This is mainly because black belt practitioners are rare, and accordingly, are usually well-known in the Brazilian Jiu Jitsu community; since it takes so long to move up through the ranks, high-level practitioners form expansive social networks and rank-checking becomes simply a matter of making a few phone calls.
This happened recently near my current academy in Utah. Two practitioners in Idaho were claiming the rank of black belt in BJJ and had opened an academy under these pretenses. I remember seeing them at local tournaments wearing black belts and had heard rumors that they were faking their rank. Eventually, one of the two “black belts” on their team was submitted by a purple belt in the advanced heavyweight division. This immediately raised eyebrows from those in attendance (myself included); the “black belt” was much larger than the purple belt, and should have had a significant experience advantage. Based on his rank (which it turns out was fake) and size advantage, he was easily the favorite in the match. You can watch a video of this match here:
After this match, they were outed almost immediately, and the resulting backlash from the BJJ community was overwhelming. A site was dedicated to educating the public about the head coach’s fraudulent BJJ credentials, and he was essentially ostracized from the BJJ community. Because of this, he is no longer eligible to compete at officially sanctioned BJJ tournaments, nor is he likely to be accepted into any credible BJJ academies in which he could earn a legitimate rank. You can listen to an in-depth discussion of this case on the Fightwork Podcast episode #248.
Moral of the story: train hard and earn your rank.