BJJ black belt can be likened to a TKD 4th dan. They are both belts where you can open your own school legitimately, they are the first belt when you earn the title of "Master", and they take roughly the same amount of time to achieve, if you train very hard and with dedication. Different arts have different belt systems, so it's silly to compare them. It doesn't make one art "better". In TKD, for example, you usually reach black belt in 3-4 years of consistent training from a good instructor. In Korea, they give them away even sooner. For 4th dan (and very few people reach this), you need around 10 years. This is where you REALLY start to teach, this is where you REALLY start developing on your own. And all 4th dans certified by Kukkiwon I've met were nothing to sneer at.