Well, I don't see why a dojo would allow students to test or ask to test for all the lower belts but then they have to wait until they're told by their sensei that they can test for black belt. As some people have pointed out, at their dojo you're told when you can test for every belt, not just black but every belt. That would make sense but I don't see why it would be just the black belt test that a student has to be told they can test. If a student does have to be told when they can test than I would think it would apply to all the belts, not just black. If a student signs up or asks to sign up for tests I would think it would apply to all the belts including black.