Well the knowledge and skill for black belt should be higher than brown belt but not much higher, after all you're not going to try to teach somebody calculus right after they've learned basic addition. For black belt to be much harder than brown belt, I see that as not being balanced and from what I've learned in the martial arts, much of it is about balance, both physical balance and balance in your whole life. For black belt to be much harder than brown belt, that would contradict the lesson in balance.
That being said, it shouldn't be disrespectful to ask about testing for the black belt, if your dojo has tests, if you're a brown belt. When you're a brown belt you're at the bridge to black belt so it's time to cross it. After all, you're asking to test for the black belt as a brown belt, not as a white belt or a green belt.